Docker Development

Patterns for building, testing, and deploying Docker containers.

Core Principles

1. Minimal images -- Alpine/distroless, multi-stage builds
2. Security first -- Non-root USER, no secrets in layers, pin versions
3. Testable -- Verifiable in CI with entrypoint bypass and DNS mocking
4. Cache-efficient -- Copy dependency files first, clean in same layer

Quick Reference

Multi-Stage Build (Node.js)

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

FROM node:20-alpine
RUN addgroup -g 1001 app && adduser -u 1001 -G app -D app
USER app
COPY --from=builder /app .
HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
  CMD wget -qO- http://localhost:3000/health || exit 1
CMD ["node", "server.js"]

Multi-Stage Build (Go -- scratch/distroless)

FROM golang:1.22-alpine AS builder
WORKDIR /app
COPY go.* ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 go build -o /app/server .

FROM gcr.io/distroless/static:nonroot
COPY --from=builder /app/server /server
CMD ["/server"]

Layer Optimization

RUN apt-get update && \
    apt-get install -y --no-install-recommends curl && \
    rm -rf /var/lib/apt/lists/*

Build Cache: Copy Dependency Files First

COPY package*.json ./
RUN npm ci
COPY . .

Dependency manifests before source so install layers stay cached on source-only changes.

BuildKit Secrets

RUN --mount=type=secret,id=ssh_key,dst=/root/.ssh/id_rsa git clone git@github.com:org/repo.git

Secrets in ENV/ARG/COPY persist in layer history (docker history). Use --mount=type=secret.

Docker Bake (Multi-Platform)

target "app" {
  platforms = ["linux/amd64", "linux/arm64"]
  cache-from = ["type=gha"]
  cache-to = ["type=gha,mode=max"]
}

Security Anti-Patterns

Anti-pattern	Fix
FROM image:latest	Pin version: image:1.2.3-alpine
No USER directive	adduser + USER appuser
chmod 777	Use specific permissions: chmod 550
privileged: true in compose	Remove or use specific cap_add
volumes: [/:/host]	Mount only needed paths
ports: ["0.0.0.0:3000:3000"]	Bind to 127.0.0.1:3000:3000
ENV DB_PASSWORD=secret	Use --mount=type=secret or compose secrets

CI Testing Gotchas

1. Bypass entrypoint: docker run --rm --entrypoint php myimage -v
2. Mock upstream DNS: docker run --rm --add-host backend:127.0.0.1 nginx-image nginx -t
3. Compose validation: cp .env.example .env before docker compose config
4. Secret scanning: Exclude .env.example, README, docs from scanners

.dockerignore

Exclude: .git, node_modules/vendor, .env*, *.pem, *.key

Compose Essentials

• depends_on with condition: service_healthy + healthcheck with start_period for startup ordering
• networks with internal: true for database isolation from external access
• profiles: [debug] for optional services that only start with --profile debug

References

• references/ci-testing.md -- Comprehensive CI testing patterns for Docker images