⚙️ Docker Containerization

Write production-grade Dockerfiles, docker-compose configurations, multi-stage builds, and container optimization. Activate on "Dockerfile", "docker", "docker-compose", "container", "multi-stage build", "docker image", "container optimization", "docker security". NOT for Kubernetes orchestration, cloud-specific container services (ECS, Cloud Run), or CI/CD pipelines (use github-actions-pipeline-builder).

Allowed Tools

Read, Write, Edit, Bash, Grep, Glob

🤝 Pairs Great With

Devops Automator: Docker images are built, tested, and deployed through CI/CD automation pipelines
Github Actions Pipeline Builder: Container builds and registry pushes are common GitHub Actions workflow stages
Site Reliability Engineer: Container health checks, resource limits, and orchestration are SRE responsibilities
Microservices Patterns: Microservices are typically deployed as individual Docker containers

References

Docker Containerization

Write production-grade Dockerfiles with multi-stage builds, security hardening, and size optimization. Covers docker-compose for local development, image layer caching, health checks, and the patterns that separate a 2GB image from a 50MB one.

When to Use

Use for:

Writing Dockerfiles from scratch or improving existing ones
Multi-stage builds for compiled languages (Go, Rust, TypeScript)
Docker Compose for local development environments
Image size optimization (choosing base images, layer caching)
Docker security scanning and hardening
Development vs production Dockerfile patterns
Debugging container build failures
.dockerignore optimization

NOT for:

Kubernetes deployment/orchestration (different domain)
Cloud-specific container services (ECS, Cloud Run, App Runner)
CI/CD pipeline configuration (use github-actions-pipeline-builder)
Container networking beyond docker-compose
Docker Swarm

Dockerfile Decision Tree

flowchart TD
    Start[What are you building?] --> Lang{Language/runtime?}
    Lang -->|Node.js/TypeScript| Node[Node pattern]
    Lang -->|Python| Python[Python pattern]
    Lang -->|Go| Go[Go pattern]
    Lang -->|Rust| Rust[Rust pattern]
    Lang -->|Static site| Static[Static pattern]

    Node --> NQ{Need build step?}
    NQ -->|Yes, TypeScript/bundler| MultiNode[Multi-stage: build + runtime]
    NQ -->|No, plain JS| SingleNode[Single stage with slim base]

    Python --> PQ{Package manager?}
    PQ -->|pip| PipPattern[pip + venv pattern]
    PQ -->|uv| UvPattern[uv pattern — fastest]
    PQ -->|poetry| PoetryPattern[poetry export pattern]

    Go --> GoMulti[Multi-stage: build + scratch/distroless]
    Rust --> RustMulti[Multi-stage: build + debian-slim]
    Static --> StaticMulti[Multi-stage: build + nginx/caddy]

Production Patterns by Language

Node.js / TypeScript (Multi-Stage)

# Stage 1: Dependencies
FROM node:22-alpine AS deps
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci --only=production

# Stage 2: Build (TypeScript/bundler)
FROM node:22-alpine AS build
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 3: Production
FROM node:22-alpine AS production
WORKDIR /app
ENV NODE_ENV=production

# Security: non-root user
RUN addgroup -g 1001 -S nodejs && \
    adduser -S nextjs -u 1001

COPY --from=deps /app/node_modules ./node_modules
COPY --from=build /app/dist ./dist
COPY package.json ./

USER nextjs
EXPOSE 3000

HEALTHCHECK --interval=30s --timeout=3s --start-period=5s \
  CMD wget -qO- http://localhost:3000/health || exit 1

CMD ["node", "dist/index.js"]

Python (uv — Fastest)

FROM python:3.12-slim AS base

# Install uv
COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /bin/

WORKDIR /app

# Install dependencies (cached layer)
COPY pyproject.toml uv.lock ./
RUN uv sync --frozen --no-dev --no-editable

# Copy application code
COPY . .

# Non-root user
RUN useradd -r -s /bin/false appuser
USER appuser

EXPOSE 8000

HEALTHCHECK --interval=30s --timeout=3s \
  CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')"

CMD ["uv", "run", "uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]

Go (Multi-Stage → Distroless)

# Build stage
FROM golang:1.22-alpine AS build
WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /server ./cmd/server

# Production: distroless (no shell, no package manager, minimal attack surface)
FROM gcr.io/distroless/static-debian12
COPY --from=build /server /server
EXPOSE 8080
USER nonroot:nonroot
ENTRYPOINT ["/server"]

Layer Caching Strategy

flowchart TD
    subgraph "Slow to change (cache hit)"
        A[Base image] --> B[System packages]
        B --> C[Language runtime deps]
    end
    subgraph "Medium change frequency"
        C --> D[Application dependencies]
    end
    subgraph "Fast changing (cache miss OK)"
        D --> E[Application code]
        E --> F[Build step]
    end

Rule: Order Dockerfile instructions from least-frequently-changed to most-frequently-changed. Each instruction creates a layer. When a layer changes, all subsequent layers are rebuilt.

Anti-Pattern: COPY Before Dependencies

Novice:

COPY . .                    # ← Busts cache on ANY file change
RUN npm install             # ← Reinstalls everything every build

Expert:

COPY package.json package-lock.json ./  # ← Only busts on dependency changes
RUN npm ci                              # ← Cached when deps unchanged
COPY . .                                # ← Only app code changes trigger rebuild

Timeline: This has been best practice since Docker layer caching was introduced, but LLMs trained on older tutorials still generate the wrong order.

Docker Compose for Development

# docker-compose.yml
services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
      target: development          # Use a dev-specific stage
    ports:
      - "${PORT:-3000}:3000"
    volumes:
      - .:/app                     # Hot reload via bind mount
      - /app/node_modules          # Anonymous volume: don't override node_modules
    environment:
      - NODE_ENV=development
      - DATABASE_URL=postgresql://postgres:postgres@db:5432/myapp
    depends_on:
      db:
        condition: service_healthy
    develop:
      watch:                       # Docker Compose Watch (2024+)
        - action: sync
          path: ./src
          target: /app/src
        - action: rebuild
          path: package.json

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

  redis:
    image: redis:7-alpine
    ports:
      - "6379:6379"
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s

volumes:
  pgdata:

Anti-Pattern: No Health Checks

Novice: Relies on depends_on alone — but that only waits for the container to START, not for the service to be READY. Expert: Always add healthcheck to database/cache services and use condition: service_healthy in depends_on. A Postgres container that has started but hasn't finished WAL recovery will crash your app.

Image Size Optimization

Base Image	Size	Use When
`node:22`	~1.1 GB	Never in production
`node:22-slim`	~200 MB	Need apt packages
`node:22-alpine`	~130 MB	Default choice
`distroless`	~20 MB	Go/Rust compiled binaries
`scratch`	0 MB	Fully static binaries
`chainguard/*`	~10-30 MB	Security-hardened alternatives

Quick Wins

# 1. Use --no-cache for apk/apt
RUN apk add --no-cache curl

# 2. Combine RUN commands to reduce layers
RUN apt-get update && \
    apt-get install -y --no-install-recommends curl && \
    rm -rf /var/lib/apt/lists/*

# 3. Use .dockerignore aggressively
# .dockerignore:
node_modules
.git
*.md
.env*
dist
coverage
.next

Security Hardening

# 1. Non-root user (MANDATORY)
RUN addgroup -g 1001 -S appgroup && \
    adduser -S appuser -u 1001 -G appgroup
USER appuser

# 2. Read-only filesystem (in compose)
# docker-compose.yml:
#   read_only: true
#   tmpfs:
#     - /tmp

# 3. No new privileges
# docker run --security-opt no-new-privileges ...

# 4. Pin image digests for reproducibility
FROM node:22-alpine@sha256:abc123...

# 5. Scan for vulnerabilities
# docker scout quickview myimage:latest
# trivy image myimage:latest

Anti-Pattern: Running as Root

Novice: Skips the USER instruction. Everything runs as root. Expert: Running as root inside a container means a container escape gives the attacker root on the host. Always create and switch to a non-root user. Only use root for package installation in build stages. Detection: docker inspect --format='{{.Config.User}}' image:tag — if empty, it's root.

Health Check Strategy by Service Type

Principle: Liveness, Not Readiness

Docker HEALTHCHECK answers one question: "Is this process alive and minimally functional?" It does NOT answer "Are all dependencies reachable?" — that's readiness (a Kubernetes concept). Conflating them causes cascading restarts: DB goes down → every API container "fails" health check → orchestrator restarts them all → thundering herd on DB recovery.

API Services

HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
  CMD wget -qO- http://localhost:${PORT}/health || exit 1

The /health endpoint should:

Return 200 if the process can serve HTTP requests
NOT check database connectivity (that's readiness)
NOT run expensive queries or computations
Respond in <100ms — it runs every 30 seconds

// Minimal /health endpoint
app.get('/health', (req, res) => res.status(200).json({ status: 'ok' }));

If you need a richer health check for monitoring dashboards (DB status, queue depth, cache hit rate), expose it on /health/detailed and do NOT wire it to Docker HEALTHCHECK.

Compose equivalent:

healthcheck:
  test: ["CMD", "wget", "-qO-", "http://localhost:3000/health"]
  interval: 30s
  timeout: 3s
  start_period: 10s
  retries: 3

Worker / Background Job Services

Workers don't serve HTTP. Use a heartbeat file pattern:

HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
  CMD test $(find /tmp/worker-heartbeat -mmin -1 2>/dev/null | wc -l) -gt 0 || exit 1

The worker writes a timestamp file on each successful job loop iteration:

// Inside your worker loop
await processJob();
fs.writeFileSync('/tmp/worker-heartbeat', Date.now().toString());

If the heartbeat file is older than 1 minute, the worker is stuck. Checks: process is alive, event loop is not blocked, jobs are being dequeued.

Static File Servers (nginx, Caddy)

HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
  CMD wget -qO- http://localhost:80/ || exit 1

Short start period — static servers boot fast. Just check it serves a page. No /health endpoint needed.

Database Containers

Use the database's native client for health checks, not HTTP:

# PostgreSQL
healthcheck:
  test: ["CMD-SHELL", "pg_isready -U postgres"]
  interval: 10s
  timeout: 5s
  start_period: 30s    # DBs are slow to start — generous grace period
  retries: 5

# Redis
healthcheck:
  test: ["CMD", "redis-cli", "ping"]
  interval: 10s
  timeout: 3s
  retries: 5

# MySQL
healthcheck:
  test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
  interval: 10s
  timeout: 5s
  start_period: 30s
  retries: 5

Tuning Parameters

Parameter	Guidance
`interval`	30s for apps, 10s for databases. Lower = more CPU overhead and log noise.
`timeout`	3-5s. If your health check takes longer, it's too expensive.
`start_period`	How long until the first check. 5s for static, 10s for APIs, 30s for databases, 60s+ for JVM apps.
`retries`	3 for apps, 5 for databases. Too low = restarts on transient blips.

References

references/multi-stage-patterns.md — Consult for complex multi-stage builds: build caching with BuildKit, cross-compilation, monorepo Dockerfiles, Bun/Deno patterns
references/compose-patterns.md — Consult for advanced docker-compose: profiles, extends, override files, networking, secrets management, GPU passthrough

Allowed Tools​

Tags​

🤝 Pairs Great With​

References​

Docker Containerization

When to Use​

Dockerfile Decision Tree​

Production Patterns by Language​

Node.js / TypeScript (Multi-Stage)​

Python (uv — Fastest)​

Go (Multi-Stage → Distroless)​

Layer Caching Strategy​

Anti-Pattern: COPY Before Dependencies​

Docker Compose for Development​

Anti-Pattern: No Health Checks​

Image Size Optimization​

Quick Wins​

Security Hardening​

Anti-Pattern: Running as Root​

Health Check Strategy by Service Type​

Principle: Liveness, Not Readiness​

API Services​

Worker / Background Job Services​

Static File Servers (nginx, Caddy)​

Database Containers​

Tuning Parameters​

References​

Allowed Tools

Tags

🤝 Pairs Great With

References

When to Use

Dockerfile Decision Tree

Production Patterns by Language

Node.js / TypeScript (Multi-Stage)

Python (uv — Fastest)

Go (Multi-Stage → Distroless)

Layer Caching Strategy

Anti-Pattern: COPY Before Dependencies

Docker Compose for Development

Anti-Pattern: No Health Checks

Image Size Optimization

Quick Wins

Security Hardening

Anti-Pattern: Running as Root

Health Check Strategy by Service Type

Principle: Liveness, Not Readiness

API Services

Worker / Background Job Services

Static File Servers (nginx, Caddy)

Database Containers

Tuning Parameters

References