Agent-Skills.md

Agent Skills: Dockerfile Best Practices

Create and optimize Dockerfiles with BuildKit, multi-stage builds, advanced caching, and security. Use this skill whenever you need to create, modify, or optimize a Dockerfile or a Docker Compose file. Also trigger when the user discusses container images, build performance, or Docker security — even if they don't explicitly mention 'Dockerfile'.

UncategorizedID: obeone/claude-skills/dockerfile-best-practices

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obeone/claude-skills
obeoneLicense: MIT
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skills/dockerfile-best-practices/SKILL.md

Skill Metadata

Name
dockerfile-best-practices
Description
"Create and optimize Dockerfiles with BuildKit, multi-stage builds, advanced caching, and security. Use this skill whenever you need to create, modify, or optimize a Dockerfile or a Docker Compose file. Also trigger when the user discusses container images, build performance, or Docker security — even if they don't explicitly mention 'Dockerfile'."

Dockerfile Best Practices

Comprehensive guide for creating optimized, secure, and fast Docker images using modern BuildKit features.

Workflow

  1. Identify language/framework → Pick template from Language Templates
  2. Apply essential rules → Every Dockerfile must follow Essential Rules
  3. Security hardening → Non-root user, pin versions, secrets management
  4. Optimize for cache → Separate deps from code, use cache mounts
  5. Multi-stage if needed → Compiled languages or distroless runtime
  6. Add metadata → OCI labels, HEALTHCHECK, STOPSIGNAL
  7. Review → Run scripts/analyze_dockerfile.py or scripts/analyze_compose.py

Essential Rules (Always Apply)

1. BuildKit syntax directive (first line, always)

# syntax=docker/dockerfile:1

2. Pin runtime versions, NOT OS versions

# ✅ GOOD
FROM python:3.12-slim
FROM node:22-alpine
FROM golang:1-alpine

# ❌ BAD - pins OS, blocks security updates
FROM python:3.12-slim-bookworm
FROM node:22-alpine3.19

3. Cache mounts for all package managers

# pip
RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt

# npm
RUN --mount=type=cache,target=/root/.npm npm ci

# yarn
RUN --mount=type=cache,target=/root/.yarn yarn install --frozen-lockfile

# go
RUN --mount=type=cache,target=/go/pkg/mod go mod download

# cargo
RUN --mount=type=cache,target=/usr/local/cargo/registry cargo build --release

# composer
RUN --mount=type=cache,target=/tmp/cache composer install --no-dev

# maven
RUN --mount=type=cache,target=/root/.m2 mvn package -DskipTests

4. APT cache setup (before any apt operation on Debian-based images)

RUN rm -f /etc/apt/apt.conf.d/docker-clean; \
    echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache

RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
    --mount=type=cache,target=/var/lib/apt,sharing=locked \
    apt-get update && apt-get install -y --no-install-recommends curl

5. Never use ARG/ENV for secrets

# ✅ GOOD - secret mount
RUN --mount=type=secret,id=api_key \
    curl -H "Authorization: $(cat /run/secrets/api_key)" https://api.example.com

# ❌ BAD - exposed in image history
ARG API_KEY

6. Non-root user with UID/GID >10000

# Debian/Ubuntu
RUN groupadd -r -g 10001 app && useradd -r -u 10001 -g app app

# Alpine
RUN addgroup -g 10001 app && adduser -u 10001 -G app -S app

7. Use COPY, never ADD

ADD has implicit behaviors (auto-extraction, URL downloads). Always use COPY.

8. Use COPY --chown instead of separate RUN chown

# ✅ GOOD - single layer, no extra overhead
COPY --chown=app:app . .

# ❌ BAD - doubles layer size
COPY . .
RUN chown -R app:app /app

9. OCI labels for metadata

LABEL org.opencontainers.image.source="https://github.com/org/repo" \
      org.opencontainers.image.description="My application" \
      org.opencontainers.image.version="1.0.0"

10. HEALTHCHECK for long-running services

HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
  CMD wget -qO- http://localhost:8000/health || exit 1

11. Create .dockerignore

Use template from assets/dockerignore-template. Critical for build context size and security.

Language Templates

Python (with uv - Recommended)

For detailed uv patterns, see references/uv_integration.md.

# syntax=docker/dockerfile:1

FROM python:3.12-slim
COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /bin/

WORKDIR /app

# Install dependencies (cached layer)
RUN --mount=type=cache,target=/root/.cache/uv \
    --mount=type=bind,source=uv.lock,target=uv.lock \
    --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
    uv sync --locked --no-install-project

# Copy and install project
COPY --chown=app:app . .
RUN --mount=type=cache,target=/root/.cache/uv uv sync --locked

# Security
RUN groupadd -r -g 10001 app && useradd -r -u 10001 -g app app
USER app

ENV PATH="/app/.venv/bin:$PATH"
CMD ["python", "-m", "myapp"]

Node.js

# syntax=docker/dockerfile:1

FROM node:22-alpine
WORKDIR /app

COPY package.json yarn.lock ./
RUN --mount=type=cache,target=/root/.yarn \
    yarn install --frozen-lockfile --production

COPY --chown=app:app . .

RUN addgroup -g 10001 app && adduser -u 10001 -G app -S app
USER app

EXPOSE 3000
HEALTHCHECK --interval=30s CMD wget -qO- http://localhost:3000/health || exit 1
CMD ["node", "index.js"]

Go (Multi-stage)

# syntax=docker/dockerfile:1

FROM golang:1-alpine AS builder
WORKDIR /app

COPY go.mod go.sum ./
RUN --mount=type=cache,target=/go/pkg/mod go mod download

COPY . .
RUN --mount=type=cache,target=/go/pkg/mod \
    --mount=type=cache,target=/root/.cache/go-build \
    CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s" -o main

FROM gcr.io/distroless/static
COPY --from=builder /app/main /main
ENTRYPOINT ["/main"]

Rust (Multi-stage)

# syntax=docker/dockerfile:1

FROM rust:1-slim AS builder
WORKDIR /app

COPY Cargo.toml Cargo.lock ./
RUN mkdir src && echo "fn main() {}" > src/main.rs
RUN --mount=type=cache,target=/usr/local/cargo/registry \
    --mount=type=cache,target=/app/target \
    cargo build --release

COPY . .
RUN --mount=type=cache,target=/usr/local/cargo/registry \
    --mount=type=cache,target=/app/target \
    cargo build --release && cp target/release/myapp /usr/local/bin/

FROM gcr.io/distroless/cc
COPY --from=builder /usr/local/bin/myapp /myapp
ENTRYPOINT ["/myapp"]

PHP (with Composer)

# syntax=docker/dockerfile:1

FROM php:8-fpm-alpine
WORKDIR /app

COPY --from=composer:latest /usr/bin/composer /usr/bin/composer

COPY composer.json composer.lock ./
RUN --mount=type=cache,target=/tmp/cache \
    composer install --no-dev --optimize-autoloader --no-scripts

COPY --chown=app:app . .
RUN composer dump-autoload --optimize

RUN addgroup -g 10001 app && adduser -u 10001 -G app -S app
USER app

EXPOSE 9000
CMD ["php-fpm"]

Debian-based (with APT cache)

# syntax=docker/dockerfile:1

FROM debian:stable-slim

RUN rm -f /etc/apt/apt.conf.d/docker-clean; \
    echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache

WORKDIR /app

RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
    --mount=type=cache,target=/var/lib/apt,sharing=locked \
    apt-get update && apt-get install -y --no-install-recommends curl ca-certificates

COPY --chown=app:app . .

RUN groupadd -r -g 10001 app && useradd -r -u 10001 -g app app
USER app

CMD ["./app"]

Docker Compose Rules

Critical rules (always enforce):

  1. Never use version: — Deprecated since Compose V2
  2. Never use container_name: — Prevents scaling and parallel environments
  3. Never use links: — Deprecated, use networks instead
  4. Use specific image tags — Never :latest
  5. Use depends_on with condition: service_healthy — Not bare depends_on

For complete Compose guide with examples, see references/compose_best_practices.md.

Build Commands Reference

# Basic build
docker buildx build -t myapp:1.0.0 .

# With remote cache (CI/CD)
docker buildx build \
  --cache-from=type=registry,ref=registry.com/myapp:cache \
  --cache-to=type=registry,ref=registry.com/myapp:cache,mode=max \
  -t myapp:1.0.0 .

# With secrets
docker buildx build --secret id=api_key,src=./key.txt -t myapp:1.0.0 .

# Multi-platform
docker buildx build --platform linux/amd64,linux/arm64 -t myapp:1.0.0 --push .

Analysis Tools

# Analyze Dockerfile
python scripts/analyze_dockerfile.py ./Dockerfile

# Analyze docker-compose.yml
python scripts/analyze_compose.py ./docker-compose.yml

Reference Documentation

For detailed information beyond what's covered here:

| Reference | Content | |-----------|---------| | references/optimization_guide.md | BuildKit internals, caching strategies, multi-stage patterns, distroless, profiling | | references/best_practices.md | Complete checklist with impact levels, version pinning philosophy, UID/GID strategy | | references/examples.md | Real-world before/after optimization examples (13+ scenarios) | | references/uv_integration.md | Python with uv: installation methods, workspaces, multi-stage, all patterns | | references/compose_best_practices.md | Complete Compose guide: networks, volumes, secrets, dev vs prod, scaling |