docker-best-practices

Purpose

Apply Docker patterns for building optimized and secure container images.

Rules

1. Layer Optimization

principles:
  - Combine related RUN commands
  - Sort multi-line arguments alphabetically
  - Clean up in same layer

patterns: |
  # GOOD: Single layer, clean cache
  RUN apt-get update && apt-get install -y \
      curl \
      git \
      vim \
      && rm -rf /var/lib/apt/lists/*

  # BAD: Multiple layers, cache remains
  RUN apt-get update
  RUN apt-get install -y curl
  RUN apt-get install -y git

2. Multi-Stage Builds

purpose:
  - Reduce final image size
  - Separate build and runtime dependencies
  - Security (no build tools in production)

pattern: |
  # Build stage
  FROM golang:1.21 AS builder
  WORKDIR /app
  COPY go.mod go.sum ./
  RUN go mod download
  COPY . .
  RUN CGO_ENABLED=0 go build -o /app/server ./cmd/server

  # Runtime stage
  FROM gcr.io/distroless/static:nonroot
  COPY --from=builder /app/server /server
  USER nonroot:nonroot
  ENTRYPOINT ["/server"]

3. Security

principles:
  - Run as non-root user
  - Pin base image versions
  - Use minimal base images
  - Don't store secrets in images

patterns: |
  # Pin version with digest
  FROM node:20-slim@sha256:abc123...

  # Create non-root user
  RUN groupadd -r appgroup && useradd -r -g appgroup appuser
  USER appuser

  # Use secrets mount (BuildKit)
  RUN --mount=type=secret,id=npmrc,target=/root/.npmrc \
      npm install

  # .dockerignore for secrets
  # .env
  # *.pem
  # credentials.json

4. Image Size Reduction

strategies:
  - Use slim/alpine base images
  - Remove build dependencies
  - Use .dockerignore
  - Multi-stage builds

minimal_bases:
  distroless: "gcr.io/distroless/static"
  alpine: "alpine:3.19"
  slim: "debian:12-slim"

patterns: |
  # Alpine for size
  FROM python:3.12-alpine
  RUN apk add --no-cache gcc musl-dev

  # Distroless for security
  FROM gcr.io/distroless/python3
  COPY --from=builder /app /app

5. Cache Optimization

principles:
  - Order from least to most frequently changing
  - Copy dependency files first
  - Use BuildKit cache mounts

patterns: |
  # Copy dependency files first
  COPY package.json package-lock.json ./
  RUN npm ci

  # Then copy source (changes frequently)
  COPY . .
  RUN npm run build

  # BuildKit cache mount
  RUN --mount=type=cache,target=/root/.cache/pip \
      pip install -r requirements.txt

6. ENTRYPOINT vs CMD

entrypoint:
  purpose: Main executable
  form: exec form ["executable"]

cmd:
  purpose: Default arguments
  form: exec form ["arg1", "arg2"]

patterns: |
  # Fixed command with variable args
  ENTRYPOINT ["python", "app.py"]
  CMD ["--port", "8080"]

  # docker run myapp --port 3000
  # Executes: python app.py --port 3000

  # Flexible command
  CMD ["python", "app.py"]

  # docker run myapp bash
  # Executes: bash

7. Health Checks

purpose: Container health monitoring
interval: how often to check
timeout: max time for check
retries: failures before unhealthy

pattern: |
  HEALTHCHECK --interval=30s --timeout=3s --retries=3 \
      CMD curl -f http://localhost:8080/health || exit 1

8. Docker Compose

best_practices:
  - Use named volumes
  - Define networks explicitly
  - Use environment files
  - Set resource limits

pattern: |
  services:
    app:
      build:
        context: .
        target: production
      environment:
        - DATABASE_URL
      env_file:
        - .env
      ports:
        - "8080:8080"
      depends_on:
        db:
          condition: service_healthy
      deploy:
        resources:
          limits:
            cpus: "1"
            memory: 512M
      networks:
        - backend

    db:
      image: postgres:16-alpine
      volumes:
        - postgres_data:/var/lib/postgresql/data
      healthcheck:
        test: ["CMD-SHELL", "pg_isready -U postgres"]
        interval: 10s
        timeout: 5s
        retries: 5
      networks:
        - backend

  volumes:
    postgres_data:

  networks:
    backend:

9. Common Patterns by Language

nodejs: |
  FROM node:20-slim AS builder
  WORKDIR /app
  COPY package*.json ./
  RUN npm ci --only=production

  FROM gcr.io/distroless/nodejs20
  WORKDIR /app
  COPY --from=builder /app/node_modules ./node_modules
  COPY . .
  CMD ["server.js"]

python: |
  FROM python:3.12-slim AS builder
  WORKDIR /app
  RUN pip install --user -r requirements.txt

  FROM python:3.12-slim
  WORKDIR /app
  COPY --from=builder /root/.local /root/.local
  COPY . .
  ENV PATH=/root/.local/bin:$PATH
  CMD ["python", "app.py"]

go: |
  FROM golang:1.21 AS builder
  WORKDIR /app
  COPY go.* ./
  RUN go mod download
  COPY . .
  RUN CGO_ENABLED=0 go build -o /server

  FROM scratch
  COPY --from=builder /server /server
  ENTRYPOINT ["/server"]

Application

When writing Dockerfiles:

1. Always use multi-stage builds
2. Always run as non-root user
3. Always pin base image versions
4. Prefer minimal base images
5. Order layers for cache efficiency
6. Clean package caches in same layer
7. Use .dockerignore
8. Add health checks