Using Docker & Docker Compose

When to use this skill

• When the user mentions "Docker", "Container", "Image", "Volume", or "Compose".
• When the user wants to ensure a consistent development environment across machines.
• When deploying to container-based services (ECS, DigitalOcean App Platform, Render).
• When the user asks to write a Dockerfile.

Workflow

1. Dockerfile Creation:
   • Select Base Image: FROM node:20-alpine (use alpine for size).
   • Set Workdir: WORKDIR /app.
   • Install Deps: Copy package.json -> RUN npm ci.
   • Copy Code: COPY . ..
   • Build/Start: RUN npm run build (if needed) -> CMD ["npm", "start"].
2. Docker Compose:
   • Create docker-compose.yml.
   • Define services (app, db, redis).
   • Map ports (ports: ["3000:3000"]).
   • Define environment variables (env_file: .env).
3. Execution:
   • Build: docker compose build.
   • Up: docker compose up -d (detached).
   • Logs: docker compose logs -f.

Instructions

Standard Node.js Dockerfile (Production Ready)

# Stage 1: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 2: Runner
FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/package*.json ./
COPY --from=builder /app/node_modules ./node_modules
ENV NODE_ENV=production
EXPOSE 3000
CMD ["npm", "start"]

Docker Compose Example

version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - DATABASE_URL=postgres://user:pass@db:5432/mydb
    depends_on:
      - db
  
  db:
    image: postgres:15-alpine
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Best Practices

• Layer Caching: Always copy package.json and install dependencies before copying source code. This prevents re-installing modules when only code changes.
• Micro-containers: Use alpine or slim tags to minimize image size (~10x smaller).
• Security: Don't run as root (use USER node in final stage if possible).

Resources

• Docker Documentation
• Dockerfile Best Practices
• Compose File Reference