docker-essentials

Docker Essentials

When to use this skill

• Writing or optimizing a Dockerfile
• Creating docker-compose configurations
• Containerizing an application
• Setting up multi-container development environments
• Debugging container issues
• Optimizing Docker image size and build times

Dockerfile best practices

Multi-stage builds

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

# Stage 2: Production
FROM node:20-alpine AS production
WORKDIR /app
RUN addgroup -g 1001 appgroup && adduser -u 1001 -G appgroup -s /bin/sh -D appuser
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/package.json ./
USER appuser
EXPOSE 3000
CMD ["node", "dist/index.js"]

Layer optimization

# GOOD: Copy dependency files first for cache
COPY package.json package-lock.json ./
RUN npm ci
COPY . .

# BAD: Busts cache on any source change
COPY . .
RUN npm install

Security

• Use specific image tags, not latest
• Run as non-root user
• Use .dockerignore to exclude secrets, node_modules, .git
• Don't store secrets in ENV — use build secrets or runtime mounts
• Scan images: docker scout cves <image>

.dockerignore

node_modules
.git
.env*
dist
*.log
.DS_Store
coverage
.nyc_output

Docker Compose

Development setup

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
      target: development
    ports:
      - "3000:3000"
    volumes:
      - .:/app
      - /app/node_modules
    environment:
      - NODE_ENV=development
      - DATABASE_URL=postgres://user:pass@db:5432/mydb
    depends_on:
      db:
        condition: service_healthy

  db:
    image: postgres:16-alpine
    ports:
      - "5432:5432"
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb
    volumes:
      - pgdata:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user"]
      interval: 5s
      timeout: 5s
      retries: 5

  redis:
    image: redis:7-alpine
    ports:
      - "6379:6379"

volumes:
  pgdata: