Docker Containerization

Docker & Containerization

Overview

Containers should be small, secure, and reproducible. A good Dockerfile builds fast, runs lean, and has no secrets baked in.

When to Use

• Creating Dockerfiles for new projects
• Optimizing image size or build time
• Setting up docker-compose for local development
• Reviewing container security

Multi-Stage Build Pattern

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

# Stage 2: Runtime
FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
USER node
EXPOSE 3000
CMD ["node", "dist/index.js"]

Best Practices

• Small base images — use alpine or distroless, not ubuntu
• Multi-stage builds — build deps don't belong in runtime image
• Layer ordering — copy package.json before source code (cache deps layer)
• Non-root user — always USER node or USER appuser
• No secrets in image — use env vars, mounted secrets, or build args (not COPY)
• .dockerignore — exclude node_modules, .git, .env, test files
• Pin versions — node:20.11-alpine not node:latest
• Health checks — HEALTHCHECK CMD curl -f http://localhost:3000/health
• One process per container — don't run supervisor/multiple services

Image Size Targets

Docker Compose for Dev

services:
  app:
    build: .
    ports: ["3000:3000"]
    volumes: ["./src:/app/src"]  # hot reload
    env_file: .env
    depends_on:
      db: { condition: service_healthy }
  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_DB: app
      POSTGRES_PASSWORD: dev
    healthcheck:
      test: ["CMD-SHELL", "pg_isready"]
      interval: 5s

Security Checklist

• Non-root user
• No secrets in image layers
• Pinned base image versions
• Minimal packages installed (no curl/wget in prod unless needed)
• Read-only filesystem where possible
• Scan with docker scout or trivy

Integration

• magic-powers:ci-cd-pipeline — build and push images in CI
• magic-powers:infrastructure-review — review container orchestration
• magic-powers:environment-setup — containerized dev environments