Docker Skill

Build and manage Docker containers, images, and Compose files for consistent deployment.

When to Use

Use this skill when the user wants to:

• Write Dockerfiles for containerization
• Create docker-compose.yml files
• Optimize Docker image size
• Multi-stage builds
• Docker networking and volumes
• Docker secrets and configs
• Container orchestration basics

Dockerfile Best Practices

• Use official base images: Alpinized, distroless when possible
• Multi-stage builds: Separate build and runtime stages
• Leverage caching: Order COPY/ADD instructions carefully
• Minimize layers: Combine commands where possible
• Clean up: Remove unnecessary files and packages
• Non-root user: Run as non-root user in container

Docker Compose

Define services, networks, and volumes:

version: '3.8'
services:
  app:
    build: .
    environment:
      - DATABASE_URL=${DATABASE_URL}
    volumes:
      - ./data:/app/data

Multi-Stage Build Example

# Stage 1: Build
FROM node:18-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 2: Production
FROM node:18-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/index.js"]

Deliverables

• Optimized Dockerfile
• docker-compose.yml (if needed)
• .dockerignore file
• Container runtime documentation

Quality Checklist

• Image size is minimized
• Security best practices followed
• Ports and volumes are properly mapped
• Environment variables are used
• Network isolation is configured
• No secrets baked into image