Container-Based Projects Skill

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Container-Based Projects

Auto-activate when: Working with Dockerfile, docker-compose.yml, docker-compose.yaml, .dockerignore, Kubernetes manifests (*.yaml, *.yml in k8s directories), container registries, or when user mentions Docker, containers, orchestration, or deployment workflows.

Guidelines for containerized applications using Docker, Docker Compose, and orchestration tools.

Out of Scope

Infrastructure orchestration - see ansible-workflow
Kubernetes patterns - separate skill

CRITICAL: Docker Compose V2 Syntax

MUST NOT use version: field (deprecated) or docker-compose with hyphen:

# MUST NOT
version: '3.8'
services:
  app:
    image: myapp

# MUST
services:
  app:
    image: myapp

docker compose up    # MUST
docker-compose up    # MUST NOT

CRITICAL: DNS Configuration

# MUST use .internal for container DNS
services:
  app:
    environment:
      - DNS_DOMAIN=.internal

# MUST NOT use .local (conflicts with mDNS/Bonjour)

Dockerfile Core Requirements

Base Images

Use Alpine Linux for minimal attack surface and smaller images
- Example: python:3.12-alpine, node:20-alpine
MUST specify version tags for reproducible builds (MUST NOT use latest)
SHOULD use image digest pinning (SHA256) for production deployments
Use official images from trusted registries
Consider distroless images for production
If Alpine packages unavailable, use Debian Slim-based containers

# RECOMMENDED: Pin by digest for production
FROM python:3.12-alpine@sha256:abc123...

# Acceptable: Pin by tag for development
FROM python:3.12-alpine

Multi-stage Builds

Separate stages for different purposes:
- base - Common dependencies and user setup
- development - Development tools and dependencies
- production - Minimal runtime with only production dependencies
Copy only necessary artifacts to final stage
Reduces final image size and attack surface
Order commands from least to most frequently changing

Security Checklist

MUST create and use non-root users
MUST set USER directive before EXPOSE and CMD
MUST NOT include secrets in layers
MUST use .dockerignore to exclude sensitive files
SHOULD scan images for vulnerabilities regularly
SHOULD keep base images updated
MUST run as non-root user (USER directive)
MUST NOT hardcode secrets or commit credentials
MUST validate all input, even from trusted sources
MUST include health checks for orchestration
MUST use Docker secrets for sensitive data in production (NOT environment variables)
MUST set no-new-privileges:true security option

services:
  app:
    security_opt:
      - no-new-privileges:true

Project Structure Recognition

Key files: Dockerfile, docker-compose.yml, .dockerignore, .devcontainer/, Makefile, k8s/

Workflow Patterns

Before starting: Check README, Makefile, docker-compose.yml, .env files

Command hierarchy: Makefile - Project scripts - Docker commands

Layer Optimization

Reduce Layer Count

Group RUN commands to reduce layers
Use && to chain related commands
Clean up in the same layer as installation

Example: Bad vs. Good

# MUST NOT: Multiple layers
RUN apk update
RUN apk add curl
RUN apk add git
RUN rm -rf /var/cache/apk/*

# MUST: Single optimized layer
RUN apk update && \
    apk add --no-cache \
        curl \
        git && \
    rm -rf /var/cache/apk/*

Alpine APK Best Practices

Use apk add --no-cache to avoid caching package index
Maintain alphabetical order in package lists for maintainability
Remove cache in the same RUN command if not using --no-cache

RUN apk add --no-cache \
        bash \
        curl \
        git \
        openssh \
        vim

Cache Optimization

Order commands from least to most frequently changing
Copy dependency files separately before copying source code
Use BuildKit cache mounts for package managers

# MUST: Dependency layer cached separately
COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/pip \
    pip install --no-cache-dir -r requirements.txt

# Source code changes won't invalidate dependency cache
COPY app/ ./app/

Package Management Best Practices

Python UV (Modern Package Manager)

# Copy uv from official image
COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv

# Install dependencies with cache mount
COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/uv \
    uv pip install --system --no-cache -r requirements.txt

Traditional Python Pip

COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/pip \
    pip install --no-cache-dir -r requirements.txt

Non-Root User Setup

# Create non-root user
ARG PUID=1000
ARG PGID=1000
ARG USER=appuser

RUN addgroup -g ${PGID} ${USER} && \
    adduser -D -u ${PUID} -G ${USER} -s /bin/sh ${USER}

# Switch to non-root user before EXPOSE and CMD
USER ${USER}

12-Factor App Compliance

| Factor | Implementation | |--------|---------------| | Configuration | Environment variables only, MUST NOT hardcode | | Dependencies | Explicit declarations with lockfiles | | Stateless | No local state, horizontally scalable | | Port Binding | Self-contained, exports via port binding | | Disposability | Fast startup/shutdown, graceful termination | | Dev/Prod Parity | Keep environments similar |

Resource Limits

MUST define resource limits for production deployments:

services:
  app:
    deploy:
      resources:
        limits:
          cpus: '1.0'
          memory: 512M
          pids: 100
        reservations:
          cpus: '0.25'
          memory: 128M

Log Rotation

MUST configure log rotation to prevent disk exhaustion:

services:
  app:
    logging:
      driver: "json-file"
      options:
        max-size: "10m"
        max-file: "3"

Complete Multi-Stage Build Template

# Base stage with common setup
FROM python:3.12-alpine AS base

# Install uv for fast dependency management
COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv

# Build arguments
ARG PUID=1000
ARG PGID=1000
ARG USER=appuser
ARG WORKDIR=/app

# Create non-root user
RUN addgroup -g ${PGID} ${USER} && \
    adduser -D -u ${PUID} -G ${USER} -s /bin/sh ${USER}

WORKDIR ${WORKDIR}

# Development stage
FROM base AS development

# Install development tools
RUN apk add --no-cache \
        bash \
        curl \
        git \
        vim

# Install development dependencies
COPY requirements.txt requirements-dev.txt ./
RUN --mount=type=cache,target=/root/.cache/uv \
    uv pip install --system --no-cache -r requirements-dev.txt

# Copy source code
COPY --chown=${USER}:${USER} . .

USER ${USER}

CMD ["python", "run.py"]

# Production stage
FROM base AS production

# Install runtime system dependencies
RUN apk add --no-cache \
        bash \
        curl

# Install production dependencies only
COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/uv \
    uv pip install --system --no-cache -r requirements.txt

# Copy application code
COPY --chown=${USER}:${USER} app/ ./app/
COPY --chown=${USER}:${USER} run.py .

# Switch to non-root user
USER ${USER}

# Health check
HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
    CMD curl -f http://localhost:8000/health || exit 1

# Expose port
EXPOSE 8000

# Run application
CMD ["python", "run.py"]

Health Checks

# HTTP service
HEALTHCHECK --interval=30s --timeout=10s --retries=3 \
  CMD curl -f http://localhost:8000/health || exit 1

# Database
HEALTHCHECK --interval=30s --timeout=10s --retries=3 \
  CMD pg_isready -U postgres || exit 1

services:
  app:
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
      interval: 30s
      timeout: 10s
      retries: 3
      start_period: 40s

Docker Compose File Organization

Use include directive for modular Compose files:

project/
├── docker-compose.yml
├── compose/
│   ├── dev.yml
│   ├── service1.yml
│   └── service2.yml
├── .env
├── .env.development
└── .env.production

# docker-compose.yml - Main compose file with includes
include:
  - compose/service1.yml
  - compose/service2.yml

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
      target: production
    environment:
      - DATABASE_URL=${DATABASE_URL}
      - LOG_LEVEL=${LOG_LEVEL:-info}
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
      interval: 30s
      timeout: 10s
      retries: 3
      start_period: 40s
    restart: unless-stopped
    depends_on:
      db:
        condition: service_healthy
    networks:
      - app_network
    security_opt:
      - no-new-privileges:true
    deploy:
      resources:
        limits:
          cpus: '1.0'
          memory: 512M
          pids: 100
    logging:
      driver: "json-file"
      options:
        max-size: "10m"
        max-file: "3"

  db:
    image: postgres:15-alpine
    environment:
      - POSTGRES_PASSWORD=${DB_PASSWORD}
    volumes:
      - db_data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD", "pg_isready", "-U", "postgres"]
      interval: 10s
      timeout: 5s
      retries: 5
    networks:
      - app_network

volumes:
  db_data:

networks:
  app_network:
    driver: bridge

Docker Secrets

MUST use Docker secrets over environment variables for sensitive data:

services:
  app:
    secrets:
      - db_password
      - api_key
    environment:
      - DB_PASSWORD_FILE=/run/secrets/db_password

secrets:
  db_password:
    file: ./secrets/db_password.txt
  api_key:
    external: true

# Read secret in application
RUN --mount=type=secret,id=db_password \
    cat /run/secrets/db_password > /app/config/db_password

Development Workflow

| Aspect | Development | Production | |--------|-------------|------------| | Base Image | Full OS for debugging | Alpine (minimal) | | Code Mount | Volume mount for hot reload | Copied into image | | Dependencies | Include dev tools | Runtime only | | Ports | Exposed for debugging | Necessary only | | Restart | no (manual control) | unless-stopped | | Logging | DEBUG | INFO/WARN |

# compose/dev.yml
services:
  app:
    build:
      target: development
    volumes:
      - .:/workspace:cached
    command: python run.py --reload
    environment:
      - ENVIRONMENT=development
      - LOG_LEVEL=debug

DevContainer Configuration

{
  "name": "Project Dev Container",
  "dockerComposeFile": "../docker-compose.yml",
  "service": "dev",
  "workspaceFolder": "/workspace",
  "customizations": {
    "vscode": {
      "extensions": [
        "ms-python.python",
        "ms-azuretools.vscode-docker"
      ]
    }
  },
  "postCreateCommand": "pip install -e .[dev]",
  "remoteUser": "appuser"
}

Makefile Integration Pattern

.PHONY: dev build up down logs
dev:
	@docker compose -f docker-compose.yml -f compose/dev.yml up
build:
	@docker compose build
up:
	@docker compose up -d
down:
	@docker compose down
logs:
	@docker compose logs -f

# Pattern rules
run-%:
	@docker compose -f docker-compose.yml -f compose/$*.yml up
stop-%:
	@docker compose stop $*

Environment Variables

Build-time Variables (ARG)

Use ARG for build-time configuration
Common ARGs: PUID, PGID, USER, WORKDIR, VERSION

ARG PYTHON_VERSION=3.12
ARG PUID=1000
ARG PGID=1000
ARG USER=appuser
ARG WORKDIR=/app

Runtime Variables (ENV)

Use ENV for runtime environment variables
Provide sensible defaults
Document required vs. optional variables

ENV PYTHONUNBUFFERED=1 \
    PYTHONDONTWRITEBYTECODE=1 \
    APP_ENV=production

.dockerignore Best Practices

Comprehensive .dockerignore organized by category to exclude unnecessary files from build context:

# Git
.git/
.gitignore
.gitattributes

# Documentation
README.md
docs/
*.md

# CI/CD
.github/
.gitlab-ci.yml

# Development
.vscode/
.idea/
.devcontainer/

# Python
__pycache__/
*.pyc
*.pyo
*.pyd
.pytest_cache/
.coverage
htmlcov/
.venv/
venv/
*.egg-info/

# Environment files
.env
.env.*

# Build artifacts
build/
dist/

# Testing
tests/
.spec/

# Logs
*.log

# System
.DS_Store
.windows
Thumbs.db

# Docker files
docker-compose*.yml
Dockerfile*
.dockerignore

# Node (if applicable)
node_modules/

# Misc
*.tmp
.cache/

Network Configuration

networks:
  app_network:
    driver: bridge
  db_network:
    driver: bridge
    internal: true  # No external access

Language-Specific Patterns

Flask Application

FROM python:3.12-alpine AS production

COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv

ARG USER=appuser
RUN adduser -D -s /bin/sh ${USER}

WORKDIR /app

COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/uv \
    uv pip install --system --no-cache -r requirements.txt

COPY --chown=${USER}:${USER} app/ ./app/
COPY --chown=${USER}:${USER} run.py .

USER ${USER}

EXPOSE 5000

HEALTHCHECK CMD curl -f http://localhost:5000/health || exit 1

CMD ["python", "run.py"]

Background Worker

FROM python:3.12-alpine AS production

COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv

ARG USER=worker
RUN adduser -D -s /bin/sh ${USER}

WORKDIR /app

COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/uv \
    uv pip install --system --no-cache -r requirements.txt

COPY --chown=${USER}:${USER} worker/ ./worker/

USER ${USER}

# No EXPOSE needed for background workers
# No HEALTHCHECK - use orchestrator health checks

CMD ["python", "-m", "worker.main"]

BuildKit Features

Cache Mounts

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

# Cache npm packages
RUN --mount=type=cache,target=/root/.npm \
    npm install

Multi-platform Builds

# Support ARM and AMD architectures
FROM --platform=$BUILDPLATFORM python:3.12-alpine

ARG TARGETPLATFORM
ARG BUILDPLATFORM

RUN echo "Building for $TARGETPLATFORM on $BUILDPLATFORM"

Multi-platform Build Guidance

Use docker buildx for building multiple architectures
Reference build platform variables: $BUILDPLATFORM, $TARGETPLATFORM
Leverage BuildKit cache exports for CI/CD: --cache-to=type=registry and --cache-from=type=registry
Pin base image versions to exact patches for consistent caching

Signal Handling and Entrypoint Scripts

Production Containers

Use gosu with exec in production entrypoint scripts to drop privileges and forward signals
Ensure CMD uses direct command execution (not shell wrapping) for proper signal delivery
When docker.sock is mounted, fix permissions in entrypoint with: chown ${USER}:${USER} /var/run/docker.sock >/dev/null 2>&1 || true

Development Containers

Sudo is acceptable for devcontainer usage

Production Entrypoint Script with Signal Handling

#!/bin/bash
set -o errexit   # abort on nonzero exitstatus
set -o nounset   # abort on unbound variable
set -o pipefail  # do not hide errors within pipes
if [ -v DOCKER_ENTRYPOINT_DEBUG ] && [ "$DOCKER_ENTRYPOINT_DEBUG" == 1 ]; then
    set -x
    set -o xtrace
fi

# If running as root, adjust the ${USER} user's UID/GID and drop to that user
if [ "$(id -u)" = "0" ]; then
    groupmod -o -g ${PGID:-1000} ${USER} 2>&1 >/dev/null|| true
    usermod -o -u ${PUID:-1000} ${USER} 2>&1 >/dev/null|| true

    # Ensure docker.sock is owned by the target user when running as root
    chown ${USER}:${USER} /var/run/docker.sock >/dev/null 2>&1 || true

    echo "Running as user ${USER}: $@"
    exec gosu ${USER} "$@"
fi

echo "Running: $@"
exec "$@"

Common Container Patterns

services:
  app:
    depends_on:
      db:
        condition: service_healthy
    deploy:
      resources:
        limits:
          cpus: '1.0'
          memory: 512M
          pids: 100

  db:
    volumes:
      - db_data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD", "pg_isready"]
      interval: 10s
      timeout: 5s
      retries: 5

volumes:
  db_data:

Git Submodules in Containers

RUN git clone --recurse-submodules https://github.com/user/repo.git
# Or: RUN git submodule update --init --recursive

Essential Docker Commands

# Service management
docker compose up / up -d / down / down -v
docker compose restart app
docker compose build / build --no-cache

# Monitoring
docker compose ps / logs -f / logs -f app
docker stats

# Execute commands
docker compose exec app sh
docker compose run --rm app pytest

# Cleanup
docker image prune -a
docker system prune

Quick Reference

Before running containers:

Check README and Makefile
Review docker-compose.yml dependencies
Check for .env.example
Understand dev vs production configs

Common mistakes:

Using version: field or docker-compose with hyphen
Running as root user
Using large base images (not Alpine)
Committing secrets
Using .local domain
Skipping health checks
Using env vars instead of secrets for sensitive data
Missing resource limits in production
No log rotation configured

Note: Container projects vary in complexity. MUST check project-specific documentation before making changes to Docker configurations.

Agent Skills: Container-Based Projects

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