docker-development

name	docker-development
description	Use when working with ANY Docker task: writing Dockerfiles, configuring docker-compose/compose.yml, multi-stage builds, docker-bake.hcl, container security audits, .dockerignore optimization, or CI/CD container testing. Triggers on: Dockerfile, docker-compose, container, image build, multi-stage, docker bake, compose.
license	(MIT AND CC-BY-SA-4.0)
compatibility	Requires docker, docker compose.
metadata	{"version":"1.14.0","repository":"https://github.com/netresearch/docker-development-skill","author":"Netresearch DTT GmbH"}
allowed-tools	["Bash(docker:*)","Bash(grep:*)","Read","Write","Glob","Grep"]

Docker Development

Core Principles

1. Minimal -- Alpine/distroless, multi-stage
2. Secure -- Non-root USER, no layer secrets, pin versions
3. Testable -- entrypoint bypass, DNS mocking
4. Cache-efficient -- deps first, clean in-layer

Quick Reference

Multi-Stage Build (Node.js)

FROM node:24-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .

FROM node:24-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:1.26-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 . .

Manifests before source keeps install layers cached.

BuildKit Secrets

RUN --mount=type=secret,id=ssh_key,dst=/root/.ssh/id_rsa git clone [e-mail]:org/repo.git

ARG leaks via docker history and SLSA provenance -- references/build-secret-leaks.md

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
5. Root-owned artifacts: bind-mount dirs (EACCES) -- references/bind-mount-ownership.md

.dockerignore

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

Compose Essentials

• startup ordering: depends_on.condition: service_healthy + healthcheck start_period
• networks.internal: true isolates databases
• profiles: [debug]: start only with --profile debug

References

• references/ci-testing.md -- CI testing patterns for Docker images
• references/dind-testing-patterns.md -- Docker-in-Docker testing patterns
• references/bind-mount-ownership.md -- root-owned bind-mount artifacts
• references/gpg-verification.md -- gpgv patterns; stale keybox locks
• references/registry-catalogue-and-pin-rot.md -- catalogue probes; pin rot
• references/build-secret-leaks.md -- ARG in provenance
• references/php-fpm-worker-starvation.md -- keepalive pins php-fpm