Docker & Containerization

Context

Apply when containerizing applications or setting up Docker-based development/deployment environments.

Instructions

1. Multi-stage builds: Always use multi-stage Dockerfiles to minimize image size.
2. Base images: Use specific version tags (not latest).
3. Layer ordering: Order Dockerfile instructions from least to most frequently changed for better caching.
4. Security:
   • Run as non-root user
   • Don't copy .env files into images
   • Use .dockerignore
5. docker-compose: Use for local development with all service dependencies.
6. Health checks: Add health check endpoints and Docker HEALTHCHECK directives.

Examples

Multi-stage Go Dockerfile

# Build stage
FROM golang:1.22-alpine AS builder
WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 go build -o /app/server ./cmd/api

# Runtime stage
FROM alpine:3.19
RUN adduser -D -g '' appuser
COPY --from=builder /app/server /usr/local/bin/server
USER appuser
EXPOSE 8080
CMD ["server"]

docker-compose.yml

services:
  api:
    build: .
    ports:
      - "8080:8080"
    env_file: .env
    depends_on:
      db:
        condition: service_healthy
  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_DB: ${DB_NAME}
    healthcheck:
      test: ["CMD-SHELL", "pg_isready"]
      interval: 5s

Anti-patterns

• ❌ Using latest tags in production
• ❌ Running as root in containers
• ❌ Copying entire project without .dockerignore
• ❌ Single-stage builds for compiled languages