Agent-Skill-Kit

Docker Expert

Expert guidance on Containerization, Dockerfiles, and Docker Compose. Enforces best practices for security, image size, and build speed.

1. Dockerfile Best Practices

When writing a Dockerfile:

• Multi-Stage Builds: ALWAYS use multi-stage builds for compiled languages or Node.js apps to keep the final image small.

  # Build Stage
  FROM node:18-alpine AS builder
  WORKDIR /app
  COPY package*.json ./
  RUN npm ci
  COPY . .
  RUN npm run build
      
  # Production Stage
  FROM node:18-alpine
  WORKDIR /app
  COPY --from=builder /app/dist ./dist
  COPY --from=builder /app/package.json ./
  RUN npm install --production
  CMD ["npm", "start"]
  

• Layer Caching: Order instructions from least to most frequently likely to change. (Copy package.json and install deps BEFORE copying source code).
• Security:
  • Never run as root (use USER node or create a user).
  • Pin base image versions (e.g., node:18-alpine3.18).

2. Docker Compose

• Versions: Use valid Compose file syntax (no version: top-level key needed in newer specs, but 3.8 is safe if required).
• Services:
  • Use healthcheck for dependencies (e.g., waiting for DB).
  • Use .env files for secrets (never hardcode passwords).

3. Debugging Containers

• Connectivity: Suggest docker network inspect or docker compose exec app curl db:5432 to test internal networking.
• Logs: docker logs -f <container_id> is the first step.
• Shell Access: docker exec -it <container_id> /bin/sh (or /bin/bash).

4. Optimization

• This skill is aggressive about image size.
• Suggest .dockerignore to exclude node_modules, .git, Dockerfile, etc.