Docker Development

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 . .

BuildKit Secrets

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

ENV

ARG

COPY

docker history

--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

FROM image:latest

image:1.2.3-alpine

USER

adduser

USER appuser

chmod 777

chmod 550

privileged: true

cap_add

volumes: [/:/host]

ports: ["0.0.0.0:3000:3000"]

127.0.0.1:3000:3000

ENV DB_PASSWORD=secret

--mount=type=secret

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

.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