# Bake file reference

# Bake file reference

# Bake file reference

   Table of contents

---

The Bake file is a file for defining workflows that you run using `docker buildx bake`.

## File format

You can define your Bake file in the following file formats:

- HashiCorp Configuration Language (HCL)
- JSON
- YAML (Compose file)

By default, Bake uses the following lookup order to find the configuration file:

1. `compose.yaml`
2. `compose.yml`
3. `docker-compose.yml`
4. `docker-compose.yaml`
5. `docker-bake.json`
6. `docker-bake.hcl`
7. `docker-bake.override.json`
8. `docker-bake.override.hcl`

You can specify the file location explicitly using the `--file` flag:

```console
$ docker buildx bake --file ../docker/bake.hcl --print
```

If you don't specify a file explicitly, Bake searches for the file in the
current working directory. If more than one Bake file is found, all files are
merged into a single definition. Files are merged according to the lookup
order. That means that if your project contains both a `compose.yaml` file and
a `docker-bake.hcl` file, Bake loads the `compose.yaml` file first, and then
the `docker-bake.hcl` file.

If merged files contain duplicate attribute definitions, those definitions are
either merged or overridden by the last occurrence, depending on the attribute.
The following attributes are overridden by the last occurrence:

- `target.cache-to`
- `target.dockerfile-inline`
- `target.dockerfile`
- `target.outputs`
- `target.platforms`
- `target.pull`
- `target.tags`
- `target.target`

For example, if `compose.yaml` and `docker-bake.hcl` both define the `tags`
attribute, the `docker-bake.hcl` is used.

```console
$ cat compose.yaml
services:
  webapp:
    build:
      context: .
      tags:
        - bar
$ cat docker-bake.hcl
target "webapp" {
  tags = ["foo"]
}
$ docker buildx bake --print webapp
{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "foo"
      ]
    }
  }
}
```

All other attributes are merged. For example, if `compose.yaml` and
`docker-bake.hcl` both define unique entries for the `labels` attribute, all
entries are included. Duplicate entries for the same label are overridden.

```console
$ cat compose.yaml
services:
  webapp:
    build:
      context: .
      labels:
        com.example.foo: "foo"
        com.example.name: "Alice"
$ cat docker-bake.hcl
target "webapp" {
  labels = {
    "com.example.bar" = "bar"
    "com.example.name" = "Bob"
  }
}
$ docker buildx bake --print webapp
{
  "group": {
    "default": {
      "targets": [
        "webapp"
      ]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "labels": {
        "com.example.foo": "foo",
        "com.example.bar": "bar",
        "com.example.name": "Bob"
      }
    }
  }
}
```

## Syntax

The Bake file supports the following property types:

- `target`: build targets
- `group`: collections of build targets
- `variable`: build arguments and variables
- `function`: custom Bake functions

You define properties as hierarchical blocks in the Bake file.
You can assign one or more attributes to a property.

The following snippet shows a JSON representation of a simple Bake file.
This Bake file defines three properties: a variable, a group, and a target.

```json
{
  "variable": {
    "TAG": {
      "default": "latest"
    }
  },
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "dockerfile": "Dockerfile",
      "tags": ["docker.io/username/webapp:${TAG}"]
    }
  }
}
```

In the JSON representation of a Bake file, properties are objects,
and attributes are values assigned to those objects.

The following example shows the same Bake file in the HCL format:

```hcl
variable "TAG" {
  default = "latest"
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  dockerfile = "Dockerfile"
  tags = ["docker.io/username/webapp:${TAG}"]
}
```

HCL is the preferred format for Bake files.
Aside from syntactic differences,
HCL lets you use features that the JSON and YAML formats don't support.

The examples in this document use the HCL format.

## Target

A target reflects a single `docker build` invocation.
Consider the following build command:

```console
$ docker build \
  --file=Dockerfile.webapp \
  --tag=docker.io/username/webapp:latest \
  https://github.com/username/webapp
```

You can express this command in a Bake file as follows:

```hcl
target "webapp" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  context = "https://github.com/username/webapp"
}
```

The following table shows the complete list of attributes that you can assign to a target:

| Name | Type | Description |
| --- | --- | --- |
| args | Map | Build arguments |
| annotations | List | Exporter annotations |
| attest | List | Build attestations |
| cache-from | List | External cache sources |
| cache-to | List | External cache destinations |
| call | String | Specify the frontend method to call for the target. |
| context | String | Set of files located in the specified path or URL |
| contexts | Map | Additional build contexts |
| description | String | Description of a target |
| dockerfile-inline | String | Inline Dockerfile string |
| dockerfile | String | Dockerfile location |
| entitlements | List | Permissions that the build process requires to run |
| extra-hosts | List | Customs host-to-IP mapping |
| inherits | List | Inherit attributes from other targets |
| labels | Map | Metadata for images |
| matrix | Map | Define a set of variables that forks a target into multiple targets. |
| name | String | Override the target name when using a matrix. |
| no-cache-filter | List | Disable build cache for specific stages |
| no-cache | Boolean | Disable build cache completely |
| output | List | Output destinations |
| policy | List | Policies to validate build sources and metadata |
| platforms | List | Target platforms |
| pull | Boolean | Always pull images |
| secret | List | Secrets to expose to the build |
| shm-size | List | Size of/dev/shm |
| ssh | List | SSH agent sockets or keys to expose to the build |
| tags | List | Image names and tags |
| target | String | Target build stage |
| ulimits | List | Ulimit options |

### target.args

Use the `args` attribute to define build arguments for the target.
This has the same effect as passing a [--build-arg](https://docs.docker.com/reference/cli/docker/image/build/#build-arg) flag to the build command.

```hcl
target "default" {
  args = {
    VERSION = "0.0.0+unknown"
  }
}
```

You can set `args` attributes to use `null` values.
Doing so forces the `target` to use the `ARG` value specified in the Dockerfile.

```hcl
variable "GO_VERSION" {
  default = "1.20.3"
}

target "webapp" {
  dockerfile = "webapp.Dockerfile"
  tags = ["docker.io/username/webapp"]
}

target "db" {
  args = {
    GO_VERSION = null
  }
  dockerfile = "db.Dockerfile"
  tags = ["docker.io/username/db"]
}
```

### target.annotations

The `annotations` attribute lets you add annotations to images built with bake.
The key takes a list of annotations, in the format of `KEY=VALUE`.

```hcl
target "default" {
  output = [{ type = "image", name = "foo" }]
  annotations = ["org.opencontainers.image.authors=dvdksn"]
}
```

By default, the annotation is added to image manifests. You can configure the
level of the annotations by adding a prefix to the annotation, containing a
comma-separated list of all the levels that you want to annotate. The following
example adds annotations to both the image index and manifests.

```hcl
target "default" {
  output = [
    {
      type = "image"
      name = "foo"
    }
  ]
  annotations = ["index,manifest:org.opencontainers.image.authors=dvdksn"]
}
```

Read about the supported levels in
[Specifying annotation levels](https://docs.docker.com/build/building/annotations/#specifying-annotation-levels).

### target.attest

The `attest` attribute lets you apply [build attestations](https://docs.docker.com/build/attestations/) to the target.
This attribute accepts the long-form CSV version of attestation parameters.

```hcl
target "default" {
  attest = [
    {
      type = "provenance"
      mode = "max"
    },
    {
      type = "sbom"
    }
  ]
}
```

### target.cache-from

Build cache sources.
The builder imports cache from the locations you specify.
It uses the [Buildx cache storage backends](https://docs.docker.com/build/cache/backends/),
and it works the same way as the [--cache-from](https://docs.docker.com/reference/cli/docker/buildx/build/#cache-from) flag.
This takes a list value, so you can specify multiple cache sources.

```hcl
target "app" {
  cache-from = [
    {
      type = "s3"
      region = "eu-west-1"
      bucket = "mybucket"
    },
    {
      type = "registry"
      ref = "user/repo:cache"
    }
  ]
}
```

### target.cache-to

Build cache export destinations.
The builder exports its build cache to the locations you specify.
It uses the [Buildx cache storage backends](https://docs.docker.com/build/cache/backends/),
and it works the same way as the [--cache-toflag](https://docs.docker.com/reference/cli/docker/buildx/build/#cache-to).
This takes a list value, so you can specify multiple cache export targets.

```hcl
target "app" {
  cache-to = [
    {
      type = "s3"
      region = "eu-west-1"
      bucket = "mybucket"
    },
    {
      type = "inline"
    }
  ]
}
```

### target.call

Specifies the frontend method to use. Frontend methods let you, for example,
execute build checks only, instead of running a build. This is the same as the
`--call` flag.

```hcl
target "app" {
  call = "check"
}
```

Supported values are:

- `build` builds the target (default)
- `check`: evaluates [build checks](https://docs.docker.com/build/checks/) for the target
- `outline`: displays the target's build arguments and their default values if available
- `targets`: lists all Bake targets in the loaded definition, along with its [description](#targetdescription).

For more information about frontend methods, refer to the CLI reference for
[docker buildx build --call](https://docs.docker.com/reference/cli/docker/buildx/build/#call).

### target.context

Specifies the location of the build context to use for this target.
Accepts a URL or a directory path.
This is the same as the [build context](https://docs.docker.com/reference/cli/docker/buildx/build/#build-context) positional argument
that you pass to the build command.

```hcl
target "app" {
  context = "./src/www"
}
```

This resolves to the current working directory (`"."`) by default.

```console
$ docker buildx bake --print -f - <<< 'target "default" {}'
[+] Building 0.0s (0/0)
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile"
    }
  }
}
```

### target.contexts

Additional build contexts.
This is the same as the [--build-contextflag](https://docs.docker.com/reference/cli/docker/buildx/build/#build-context).
This attribute takes a map, where keys result in named contexts that you can
reference in your builds.

You can specify different types of contexts, such local directories, Git URLs,
and even other Bake targets. Bake automatically determines the type of
a context based on the pattern of the context value.

| Context type | Example |
| --- | --- |
| Container image | docker-image://alpine@sha256:0123456789 |
| Git URL | https://github.com/user/proj.git |
| HTTP URL | https://example.com/files |
| Local directory | ../path/to/src |
| Bake target | target:base |

#### Pin an image version

```hcl
# docker-bake.hcl
target "app" {
  contexts = {
    alpine = "docker-image://alpine:3.13"
  }
}
```

```Dockerfile
# Dockerfile
FROM alpine
RUN echo "Hello world"
```

#### Use a local directory

```hcl
# docker-bake.hcl
target "app" {
  contexts = {
    src = "../path/to/source"
  }
}
```

```Dockerfile
# Dockerfile
FROM scratch AS src
FROM golang
COPY --from=src . .
```

#### Use another target as base

> Note
>
> You should prefer to use regular multi-stage builds over this option. You can
> Use this feature when you have multiple Dockerfiles that can't be easily
> merged into one.

```hcl
# docker-bake.hcl
target "base" {
  dockerfile = "baseapp.Dockerfile"
}

target "app" {
  contexts = {
    baseapp = "target:base"
  }
}
```

```Dockerfile
# Dockerfile
FROM baseapp
RUN echo "Hello world"
```

### target.description

Defines a human-readable description for the target, clarifying its purpose or
functionality.

```hcl
target "lint" {
  description = "Runs golangci-lint to detect style errors"
  args = {
    GOLANGCI_LINT_VERSION = null
  }
  dockerfile = "lint.Dockerfile"
}
```

This attribute is useful when combined with the `docker buildx bake --list=targets`
option, providing a more informative output when listing the available build
targets in a Bake file.

### target.dockerfile-inline

Uses the string value as an inline Dockerfile for the build target.

```hcl
target "default" {
  dockerfile-inline = "FROM alpine\nENTRYPOINT [\"echo\", \"hello\"]"
}
```

The `dockerfile-inline` takes precedence over the `dockerfile` attribute.
If you specify both, Bake uses the inline version.

### target.dockerfile

Name of the Dockerfile to use for the build.
This is the same as the [--fileflag](https://docs.docker.com/reference/cli/docker/image/build/#file) for the `docker build` command.

```hcl
target "default" {
  dockerfile = "./src/www/Dockerfile"
}
```

Resolves to `"Dockerfile"` by default.

```console
$ docker buildx bake --print -f - <<< 'target "default" {}'
[+] Building 0.0s (0/0)
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile"
    }
  }
}
```

### target.entitlements

Entitlements are permissions that the build process requires to run.

Currently supported entitlements are:

- `network.host`: Allows the build to use commands that access the host network. In Dockerfile, use [RUN --network=host](https://docs.docker.com/reference/dockerfile/#run---networkhost) to run a command with host network enabled.
- `security.insecure`: Allows the build to run commands in privileged containers that are not limited by the default security sandbox. Such container may potentially access and modify system resources. In Dockerfile, use [RUN --security=insecure](https://docs.docker.com/reference/dockerfile/#run---security) to run a command in a privileged container.

```hcl
target "integration-tests" {
  # this target requires privileged containers to run nested containers
  entitlements = ["security.insecure"]
}
```

Entitlements are enabled with a two-step process. First, a target must declare the entitlements it requires. Secondly, when invoking the `bake` command, the user must grant the entitlements by passing the `--allow` flag or confirming the entitlements when prompted in an interactive terminal. This is to ensure that the user is aware of the possibly insecure permissions they are granting to the build process.

### target.extra-hosts

Use the `extra-hosts` attribute to define customs host-to-IP mapping for the
target. This has the same effect as passing a [--add-host](https://docs.docker.com/reference/cli/docker/buildx/build/#add-host) flag to
the build command.

```hcl
target "default" {
  extra-hosts = {
    my_hostname = "8.8.8.8"
  }
}
```

### target.inherits

A target can inherit attributes from other targets.
Use `inherits` to reference from one target to another.

In the following example,
the `app-dev` target specifies an image name and tag.
The `app-release` target uses `inherits` to reuse the tag name.

```hcl
variable "TAG" {
  default = "latest"
}

target "app-dev" {
  tags = ["docker.io/username/myapp:${TAG}"]
}

target "app-release" {
  inherits = ["app-dev"]
  platforms = ["linux/amd64", "linux/arm64"]
}
```

The `inherits` attribute is a list,
meaning you can reuse attributes from multiple other targets.
In the following example, the `app-release` target reuses attributes
from both the `app-dev` and `_release` targets.

```hcl
target "app-dev" {
  args = {
    GO_VERSION = "1.20"
    BUILDX_EXPERIMENTAL = 1
  }
  tags = ["docker.io/username/myapp"]
  dockerfile = "app.Dockerfile"
  labels = {
    "org.opencontainers.image.source" = "https://github.com/username/myapp"
  }
}

target "_release" {
  args = {
    BUILDKIT_CONTEXT_KEEP_GIT_DIR = 1
    BUILDX_EXPERIMENTAL = 0
  }
}

target "app-release" {
  inherits = ["app-dev", "_release"]
  platforms = ["linux/amd64", "linux/arm64"]
}
```

When inheriting attributes from multiple targets and there's a conflict,
the target that appears last in the `inherits` list takes precedence.
The previous example defines the `BUILDX_EXPERIMENTAL` argument twice for the `app-release` target.
It resolves to `0` because the `_release` target appears last in the inheritance chain:

```console
$ docker buildx bake --print app-release
[+] Building 0.0s (0/0)
{
  "group": {
    "default": {
      "targets": [
        "app-release"
      ]
    }
  },
  "target": {
    "app-release": {
      "context": ".",
      "dockerfile": "app.Dockerfile",
      "args": {
        "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
        "BUILDX_EXPERIMENTAL": "0",
        "GO_VERSION": "1.20"
      },
      "labels": {
        "org.opencontainers.image.source": "https://github.com/username/myapp"
      },
      "tags": [
        "docker.io/username/myapp"
      ],
      "platforms": [
        "linux/amd64",
        "linux/arm64"
      ]
    }
  }
}
```

### target.labels

Assigns image labels to the build.
This is the same as the `--label` flag for `docker build`.

```hcl
target "default" {
  labels = {
    "org.opencontainers.image.source" = "https://github.com/username/myapp"
    "com.docker.image.source.entrypoint" = "Dockerfile"
  }
}
```

It's possible to use a `null` value for labels.
If you do, the builder uses the label value specified in the Dockerfile.

### target.matrix

A matrix strategy lets you fork a single target into multiple different
variants, based on parameters that you specify.
This works in a similar way to [Matrix strategies for GitHub Actions].
You can use this to reduce duplication in your bake definition.

The `matrix` attribute is a map of parameter names to lists of values.
Bake builds each possible combination of values as a separate target.

Each generated target **must** have a unique name.
To specify how target names should resolve, use the `name` attribute.

The following example resolves the `app` target to `app-foo` and `app-bar`.
It also uses the matrix value to define the [target build stage](#targettarget).

```hcl
target "app" {
  name = "app-${tgt}"
  matrix = {
    tgt = ["foo", "bar"]
  }
  target = tgt
}
```

```console
$ docker buildx bake --print app
[+] Building 0.0s (0/0)
{
  "group": {
    "app": {
      "targets": [
        "app-foo",
        "app-bar"
      ]
    },
    "default": {
      "targets": [
        "app"
      ]
    }
  },
  "target": {
    "app-bar": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "target": "bar"
    },
    "app-foo": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "target": "foo"
    }
  }
}
```

#### Multiple axes

You can specify multiple keys in your matrix to fork a target on multiple axes.
When using multiple matrix keys, Bake builds every possible variant.

The following example builds four targets:

- `app-foo-1-0`
- `app-foo-2-0`
- `app-bar-1-0`
- `app-bar-2-0`

```hcl
target "app" {
  name = "app-${tgt}-${replace(version, ".", "-")}"
  matrix = {
    tgt = ["foo", "bar"]
    version = ["1.0", "2.0"]
  }
  target = tgt
  args = {
    VERSION = version
  }
}
```

#### Multiple values per matrix target

If you want to differentiate the matrix on more than just a single value,
you can use maps as matrix values. Bake creates a target for each map,
and you can access the nested values using dot notation.

The following example builds two targets:

- `app-foo-1-0`
- `app-bar-2-0`

```hcl
target "app" {
  name = "app-${item.tgt}-${replace(item.version, ".", "-")}"
  matrix = {
    item = [
      {
        tgt = "foo"
        version = "1.0"
      },
      {
        tgt = "bar"
        version = "2.0"
      }
    ]
  }
  target = item.tgt
  args = {
    VERSION = item.version
  }
}
```

### target.name

Specify name resolution for targets that use a matrix strategy.
The following example resolves the `app` target to `app-foo` and `app-bar`.

```hcl
target "app" {
  name = "app-${tgt}"
  matrix = {
    tgt = ["foo", "bar"]
  }
  target = tgt
}
```

### target.network

Specify the network mode for the whole build request. This will override the default network mode
for all the `RUN` instructions in the Dockerfile. Accepted values are `default`, `host`, and `none`.

Usually, a better approach to set the network mode for your build steps is to instead use `RUN --network=<value>`
in your Dockerfile. This way, you can set the network mode for individual build steps and everyone building
the Dockerfile gets consistent behavior without needing to pass additional flags to the build command.

If you set network mode to `host` in your Bake file, you must also grant `network.host` entitlement when
invoking the `bake` command. This is because `host` network mode requires elevated privileges and can be a security risk.
You can pass `--allow=network.host` to the `docker buildx bake` command to grant the entitlement, or you can
confirm the entitlement when prompted if you are using an interactive terminal.

```hcl
target "app" {
  # make sure this build does not access internet
  network = "none"
}
```

### target.no-cache-filter

Don't use build cache for the specified stages.
This is the same as the `--no-cache-filter` flag for `docker build`.
The following example avoids build cache for the `foo` build stage.

```hcl
target "default" {
  no-cache-filter = ["foo"]
}
```

### target.no-cache

Don't use cache when building the image.
This is the same as the `--no-cache` flag for `docker build`.

```hcl
target "default" {
  no-cache = true
}
```

### target.output

Configuration for exporting the build output.
This is the same as the [--outputflag](https://docs.docker.com/reference/cli/docker/buildx/build/#output).
The following example configures the target to use a cache-only output,

```hcl
target "default" {
  output = [{ type = "cacheonly" }]
}
```

### target.policy

Policies to validate build sources and metadata. Each entry uses the same keys
as the `--policy` flag for `docker buildx build` (`filename`, `reset`,
`disabled`, `strict`, `log-level`). Bake also automatically loads
`Dockerfile.rego` alongside the target Dockerfile when present.

```hcl
target "default" {
  policy = [
    { filename = "extra.rego" },
  ]
}
```

### target.platforms

Set target platforms for the build target.
This is the same as the [--platformflag](https://docs.docker.com/reference/cli/docker/buildx/build/#platform).
The following example creates a multi-platform build for three architectures.

```hcl
target "default" {
  platforms = ["linux/amd64", "linux/arm64", "linux/arm/v7"]
}
```

### target.pull

Configures whether the builder should attempt to pull images when building the target.
This is the same as the `--pull` flag for `docker build`.
The following example forces the builder to always pull all images referenced in the build target.

```hcl
target "default" {
  pull = true
}
```

### target.secret

Defines secrets to expose to the build target.
This is the same as the [--secretflag](https://docs.docker.com/reference/cli/docker/buildx/build/#secret).

```hcl
variable "HOME" {
  default = null
}

target "default" {
  secret = [
    {
      type = "env"
      id = "KUBECONFIG"
    },
    {
      type = "file"
      id = "aws"
      src = "${HOME}/.aws/credentials"
    }
  ]
}
```

This lets you [mount the secret](https://docs.docker.com/reference/dockerfile/#run---mounttypesecret) in your Dockerfile.

```dockerfile
RUN --mount=type=secret,id=aws,target=/root/.aws/credentials \
    aws cloudfront create-invalidation ...
RUN --mount=type=secret,id=KUBECONFIG,env=KUBECONFIG \
    helm upgrade --install
```

### target.shm-size

Sets the size of the shared memory allocated for build containers when using
`RUN` instructions.

The format is `<number><unit>`. `number` must be greater than `0`. Unit is
optional and can be `b` (bytes), `k` (kilobytes), `m` (megabytes), or `g`
(gigabytes). If you omit the unit, the system uses bytes.

This is the same as the `--shm-size` flag for `docker build`.

```hcl
target "default" {
  shm-size = "128m"
}
```

> Note
>
> In most cases, it is recommended to let the builder automatically determine
> the appropriate configurations. Manual adjustments should only be considered
> when specific performance tuning is required for complex build scenarios.

### target.ssh

Defines SSH agent sockets or keys to expose to the build.
This is the same as the [--sshflag](https://docs.docker.com/reference/cli/docker/buildx/build/#ssh).
This can be useful if you need to access private repositories during a build.

```hcl
target "default" {
  ssh = [{ id = "default" }]
}
```

```dockerfile
FROM alpine
RUN --mount=type=ssh \
    apk add git openssh-client \
    && install -m 0700 -d ~/.ssh \
    && ssh-keyscan github.com >> ~/.ssh/known_hosts \
    && git clone git@github.com:user/my-private-repo.git
```

### target.tags

Image names and tags to use for the build target.
This is the same as the [--tagflag](https://docs.docker.com/reference/cli/docker/image/build/#tag).

```hcl
target "default" {
  tags = [
    "org/repo:latest",
    "myregistry.azurecr.io/team/image:v1"
  ]
}
```

### target.target

Set the target build stage to build.
This is the same as the [--targetflag](https://docs.docker.com/reference/cli/docker/image/build/#target).

```hcl
target "default" {
  target = "binaries"
}
```

### target.ulimits

Ulimits overrides the default ulimits of build's containers when using `RUN`
instructions and are specified with a soft and hard limit as such:
`<type>=<soft limit>[:<hard limit>]`, for example:

```hcl
target "app" {
  ulimits = [
    "nofile=1024:1024"
  ]
}
```

> Note
>
> If you do not provide a `hard limit`, the `soft limit` is used
> for both values. If no `ulimits` are set, they are inherited from
> the default `ulimits` set on the daemon.

> Note
>
> In most cases, it is recommended to let the builder automatically determine
> the appropriate configurations. Manual adjustments should only be considered
> when specific performance tuning is required for complex build scenarios.

## Group

Groups allow you to invoke multiple builds (targets) at once.

```hcl
group "default" {
  targets = ["db", "webapp-dev"]
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
}

target "db" {
  dockerfile = "Dockerfile.db"
  tags = ["docker.io/username/db"]
}
```

Groups take precedence over targets, if both exist with the same name.
The following bake file builds the `default` group.
Bake ignores the `default` target.

```hcl
target "default" {
  dockerfile-inline = "FROM ubuntu"
}

group "default" {
  targets = ["alpine", "debian"]
}
target "alpine" {
  dockerfile-inline = "FROM alpine"
}
target "debian" {
  dockerfile-inline = "FROM debian"
}
```

## Variable

The HCL file format supports variable block definitions.
You can use variables as build arguments in your Dockerfile,
or interpolate them in attribute values in your Bake file.

```hcl
variable "TAG" {
  type = string
  default = "latest"
  description = "Tag to use for build"
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:${TAG}"]
}
```

You can assign a default value for a variable in the Bake file,
or assign a `null` value to it. If you assign a `null` value,
Buildx uses the default value from the Dockerfile instead.

You can also add a description of the variable's purpose with the `description` field. This attribute is useful when combined with the `docker buildx bake --list=variables` option, providing a more informative output when listing the available variables in a Bake file.

You can override variable defaults set in the Bake file using environment variables.
The following example sets the `TAG` variable to `dev`,
overriding the default `latest` value shown in the previous example.

```console
$ TAG=dev docker buildx bake webapp-dev
```

Variables can also be assigned an explicit type.
If provided, it will be used to validate the default value (if set), as well as any overrides.
This is particularly useful when using complex types which are intended to be overridden.
The previous example could be expanded to apply an arbitrary series of tags.

```hcl
variable "TAGS" {
  default = ["latest"]
  type = list(string)
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = [for tag in TAGS: "docker.io/username/webapp:${tag}"]
}
```

This example shows how to generate three tags without changing the file
or using custom functions/parsing:

```console
$ TAGS=dev,latest,2 docker buildx bake webapp-dev
```

### Variable typing

The following primitive types are available:

- `string`
- `number`
- `bool`

The type is expressed like a keyword; it must be expressed as a literal:

```hcl
variable "OK" {
  type = string
}

# cannot be an actual string
variable "BAD" {
  type = "string"
}

# cannot be the result of an expression
variable "ALSO_BAD" {
  type = lower("string")
}
```

Specifying primitive types can be valuable to show intent (especially when a default is not provided),
but bake will generally behave as expected without explicit typing.

Complex types are expressed with "type constructors"; they are:

- `tuple([<type>,...])`
- `list(<type>)`
- `set(<type>)`
- `map(<type>)`
- `object({<attr>=<type>},...})`

The following are examples of each of those, as well as how the (optional) default value would be expressed:

```hcl
# structured way to express "1.2.3-alpha"
variable "MY_VERSION" {
  type = tuple([number, number, number, string])
  default = [1, 2, 3, "alpha"]
}

# JDK versions used in a matrix build
variable "JDK_VERSIONS" {
  type = list(number)
  default = [11, 17, 21]
}

# better way to express the previous example; this will also
# enforce set semantics and allow use of set-based functions
variable "JDK_VERSIONS" {
  type = set(number)
  default = [11, 17, 21]
}

# with the help of lookup(), translate a 'feature' to a tag
variable "FEATURE_TO_NAME" {
  type = map(string)
  default = {featureA = "slim", featureB = "tiny"}
}

# map a branch name to a registry location
variable "PUSH_DESTINATION" {
  type = object({branch = string, registry = string})
  default = {branch = "main", registry = "prod-registry.invalid.com"}
}

# make the previous example more useful with composition
variable "PUSH_DESTINATIONS" {
  type = list(object({branch = string, registry = string}))
  default = [
    {branch = "develop", registry = "test-registry.invalid.com"},
    {branch = "main", registry = "prod-registry.invalid.com"},
  ]
}
```

Note that in each example, the default value would be valid even if typing was not present.
If typing was omitted, the first three would all be considered `tuple`;
you would be restricted to functions that operate on `tuple` and, for example, not be able to add elements.
Similarly, the third and fourth would both be considered `object`, with the limits and semantics of that type.
In short, in the absence of a type, any value delimited with `[]` is a `tuple`
and value delimited with `{}` is an `object`.
Explicit typing for complex types not only opens up the ability to use functions applicable to that specialized type,
but is also a precondition for providing overrides.

> Note
>
> See [HCL Type Expressions](https://github.com/hashicorp/hcl/tree/main/ext/typeexpr) page for more details.

### Overriding variables

As mentioned in the [intro to variables](#variable), primitive types (`string`, `number`, and `bool`)
can be overridden without typing and will generally behave as expected.
(When explicit typing is not provided, a variable is assumed to be primitive when the default value lacks `{}` or `[]` delimiters;
a variable with neither typing nor a default value is treated as `string`.)
Naturally, these same overrides can be used alongside explicit typing too;
they may help in edge cases where you want `VAR=true` to be a `string`, where without typing,
it may be a `string` or a `bool` depending on how/where it's used.
Overriding a variable with a complex type can only be done when the type is provided.
This is still done via environment variables, but the values can be provided via CSV or JSON.

#### CSV overrides

This is considered the canonical method and is well suited to interactive usage.
It is assumed that `list` and `set` will be the most common complex type,
as well as the most common complex type designed to be overridden.
Thus, there is full CSV support for `list` and `set`
(and `tuple`; despite being considered a structural type, it is more like a collection type in this regard).

There is limited support for `map` and `object` and no support for composite types;
for these advanced cases, an alternative mechanism [using JSON](#json-overrides) is available.

#### JSON overrides

Overrides can also be provided via JSON.
This is the only method available for providing some complex types and may be convenient if overrides are already JSON
(for example, if they come from a JSON API).
It can also be used when dealing with values are difficult or impossible to specify using CSV (e.g., values containing quotes or commas).
To use JSON, simply append `_JSON` to the variable name.
In this contrived example, CSV cannot handle the second value; despite being a supported CSV type, JSON must be used:

```hcl
variable "VALS" {
  type = list(string)
  default = ["some", "list"]
}
```

```console
$ cat data.json
["hello","with,comma","with\"quote"]
$ VALS_JSON=$(< data.json) docker buildx bake

# CSV equivalent, though the second value cannot be expressed at all
$ VALS='hello,"with""quote"' docker buildx bake
```

This example illustrates some precedence and usage rules:

```hcl
variable "FOO" {
  type = string
  default = "foo"
}

variable "FOO_JSON" {
  type = string
  default = "foo"
}
```

The variable `FOO` can *only* be overridden using CSV because `FOO_JSON`, which would typically used for a JSON override,
is already a defined variable.
Since `FOO_JSON` is an actual variable, setting that environment variable would be expected to a CSV value.
A JSON override *is* possible for this variable, using environment variable `FOO_JSON_JSON`.

```Console
# These three are all equivalent, setting variable FOO=bar
$ FOO=bar docker buildx bake <...>
$ FOO='bar' docker buildx bake <...>
$ FOO="bar" docker buildx bake <...>

# Sets *only* variable FOO_JSON; FOO is untouched
$ FOO_JSON=bar docker buildx bake <...>

# This also sets FOO_JSON, but will fail due to not being valid JSON
$ FOO_JSON_JSON=bar docker buildx bake <...>

# These are all equivalent
$ cat data.json
"bar"
$ FOO_JSON_JSON=$(< data.json) docker buildx bake <...>
$ FOO_JSON_JSON='"bar"' docker buildx bake <...>
$ FOO_JSON=bar docker buildx bake <...>

# This results in setting two different variables, both specified as CSV (FOO=bar and FOO_JSON="baz")
$ FOO=bar FOO_JSON='"baz"' docker buildx bake <...>

# These refer to the same variable with FOO_JSON_JSON having precedence and read as JSON (FOO_JSON=baz)
$ FOO_JSON=bar FOO_JSON_JSON='"baz"' docker buildx bake <...>
```

### Built-in variables

The following variables are built-ins that you can use with Bake without having
to define them.

| Variable | Description |
| --- | --- |
| BAKE_CMD_CONTEXT | Holds the main context when building using a remote Bake file. |
| BAKE_LOCAL_PLATFORM | Returns the current platform’s default platform specification (e.g.linux/amd64). |

### Use environment variable as default

You can set a Bake variable to use the value of an environment variable as a default value:

```hcl
variable "HOME" {
  default = "$HOME"
}
```

### Interpolate variables into attributes

To interpolate a variable into an attribute string value,
you must use curly brackets.
The following doesn't work:

```hcl
variable "HOME" {
  default = "$HOME"
}

target "default" {
  ssh = ["default=$HOME/.ssh/id_rsa"]
}
```

Wrap the variable in curly brackets where you want to insert it:

```diff
variable "HOME" {
    default = "$HOME"
  }

  target "default" {
-   ssh = ["default=$HOME/.ssh/id_rsa"]
+   ssh = ["default=${HOME}/.ssh/id_rsa"]
  }
```

Before you can interpolate a variable into an attribute,
first you must declare it in the bake file,
as demonstrated in the following example.

```console
$ cat docker-bake.hcl
target "default" {
  dockerfile-inline = "FROM ${BASE_IMAGE}"
}
$ docker buildx bake
[+] Building 0.0s (0/0)
docker-bake.hcl:2
--------------------
   1 |     target "default" {
   2 | >>>   dockerfile-inline = "FROM ${BASE_IMAGE}"
   3 |     }
   4 |
--------------------
ERROR: docker-bake.hcl:2,31-41: Unknown variable; There is no variable named "BASE_IMAGE"., and 1 other diagnostic(s)
$ cat >> docker-bake.hcl

variable "BASE_IMAGE" {
  default = "alpine"
}

$ docker buildx bake
[+] Building 0.6s (5/5) FINISHED
```

## Function

A [set of general-purpose functions](https://github.com/docker/buildx/blob/master/docs/bake-stdlib.md) provided by [go-cty](https://github.com/zclconf/go-cty/tree/main/cty/function/stdlib)
are available for use in HCL files:

```hcl
# docker-bake.hcl
target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  args = {
    buildno = "${add(123, 1)}"
  }
}
```

In addition, [user defined functions](https://github.com/hashicorp/hcl/tree/main/ext/userfunc)
are also supported:

```hcl
# docker-bake.hcl
function "increment" {
  params = [number]
  result = number + 1
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  args = {
    buildno = "${increment(123)}"
  }
}
```

> Note
>
> See [User defined HCL functions](https://docs.docker.com/build/bake/hcl-funcs/) page for more details.
