Docker Composer

Docker Composer

Creates optimized, production-ready Dockerfiles and docker-compose configurations for detected application stacks, including multi-stage builds for minimal image sizes, proper layer caching, health checks, and security hardening.

When to Use

• User asks to "dockerize this app", "create a Dockerfile", or "set up docker-compose"
• An existing Dockerfile is inefficient (large image, slow builds, running as root)
• A multi-service application needs a docker-compose setup for local development
• User wants to add a Docker setup for CI or deployment
• User asks for multi-stage builds to separate build and runtime environments
• Container image size or build time needs to be reduced

Process

1. Detect the application stack:

   • Check for package.json (Node.js), requirements.txt/pyproject.toml (Python), go.mod (Go), pom.xml/build.gradle (Java), Gemfile (Ruby), Cargo.toml (Rust)
   • Note the framework (Express, FastAPI, Spring Boot, etc.)
   • Identify if it's a static site, API server, worker process, or full-stack app

2. Choose the right base image:

   • Prefer official slim/alpine variants: node:20-slim, python:3.12-slim, golang:1.22-alpine
   • For final runtime stages, use distroless where possible (gcr.io/distroless/nodejs20-debian12)
   • Pin to a specific digest or version tag — never use latest
   • Note the trade-offs (alpine uses musl vs. glibc which can affect native modules)

3. Design the multi-stage build:

   • Stage 1 (deps/builder): install all dependencies, run build tools
   • Stage 2 (runtime): copy only the compiled artifacts and runtime dependencies
   • This keeps the final image free of build tools, source code, and dev dependencies

4. Apply layer caching optimization:

   • Copy dependency manifests (package.json, requirements.txt) BEFORE source code
   • Run dependency install BEFORE copying application source
   • This ensures the expensive install step is cached unless dependencies change

5. Security hardening:

   • Create and use a non-root user (UID 1000): RUN adduser --system appuser && USER appuser
   • Set WORKDIR explicitly
   • Use COPY --chown=appuser:appuser to set ownership
   • Avoid sudo, apt-get upgrade, or installing unnecessary packages
   • Set --no-cache for apk add / --no-install-recommends for apt-get
   • Expose only necessary ports

6. Add health check: HEALTHCHECK --interval=30s --timeout=5s CMD curl -f http://localhost:${PORT}/health || exit 1

7. For docker-compose:

   • Define services, networks, and named volumes
   • Map ports and environment variables
   • Set depends_on with condition: service_healthy for database readiness
   • Include development overrides (docker-compose.override.yml) for hot reload

8. Generate a .dockerignore to exclude node_modules, .git, test files, and local config.

Output Format

# Dockerfile
# ── Stage 1: Dependencies ──────────────────────────────────────────────────────
FROM node:20-slim AS deps
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci --only=production

# ── Stage 2: Builder ──────────────────────────────────────────────────────────
FROM node:20-slim AS builder
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci
COPY . .
RUN npm run build

# ── Stage 3: Runtime ──────────────────────────────────────────────────────────
FROM node:20-slim AS runtime
ENV NODE_ENV=production PORT=3000
WORKDIR /app

RUN addgroup --system appgroup && adduser --system --ingroup appgroup appuser

COPY --from=deps --chown=appuser:appgroup /app/node_modules ./node_modules
COPY --from=builder --chown=appuser:appgroup /app/dist ./dist
COPY --chown=appuser:appgroup package.json ./

USER appuser
EXPOSE 3000
HEALTHCHECK --interval=30s --timeout=5s --retries=3 \
  CMD curl -f http://localhost:3000/health || exit 1

CMD ["node", "dist/server.js"]

Examples

Example Input

Dockerize a FastAPI Python app. It needs PostgreSQL and Redis.
Dev setup with hot reload. Production build should be minimal.

Example Output (summary)

Files generated:
  Dockerfile           — multi-stage: python:3.12-slim builder → distroless runtime
  docker-compose.yml   — services: app, postgres:16, redis:7-alpine
                          volumes: postgres_data, redis_data
                          networks: backend (internal), frontend (exposed)
  docker-compose.override.yml — mounts ./src as volume, runs uvicorn --reload
  .dockerignore        — excludes __pycache__, .venv, .git, tests/, *.pyc

docker-compose.yml services:
  app:
    build: .
    ports: ["8000:8000"]
    env_file: .env
    depends_on:
      postgres: { condition: service_healthy }
      redis: { condition: service_healthy }
  postgres:
    image: postgres:16-alpine
    healthcheck: pg_isready
    volumes: [postgres_data:/var/lib/postgresql/data]
  redis:
    image: redis:7-alpine
    healthcheck: redis-cli ping

Boundaries

• Do NOT include secrets, passwords, or API keys in Dockerfiles or docker-compose.yml — always use environment variables and .env files (which should be in .gitignore).
• Do NOT use latest tag for base images in production Dockerfiles — always pin versions.
• Do NOT run container processes as root unless absolutely necessary (and flag it clearly if required).
• Do NOT use ADD with remote URLs — use COPY for local files and curl/wget in a RUN step.
• Warn if the application has native module dependencies that may be incompatible with Alpine's musl libc.
• Do NOT generate Kubernetes manifests from this skill — recommend the api-scaffolder or dedicated K8s tooling.
• Always generate a .dockerignore alongside the Dockerfile.