---
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: "1.10.0"
  repository: "https://github.com/netresearch/docker-development-skill"
  author: "Netresearch DTT GmbH"
allowed-tools:
  - "Bash(docker:*)"
  - "Bash(grep:*)"
  - "Read"
  - "Write"
  - "Glob"
  - "Grep"
---

# Docker Development

Patterns for building, testing, and deploying Docker containers.

## Core Principles

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

## Quick Reference

### Multi-Stage Build (Node.js)

```dockerfile
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)

```dockerfile
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

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

### Build Cache: Copy Dependency Files First

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

Manifests before source keeps install layers cached on source-only changes.

### BuildKit Secrets

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

`ENV`/`ARG`/`COPY` secrets persist in `docker history`. Use `--mount=type=secret`.

### Docker Bake (Multi-Platform)

```hcl
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
5. **Root-owned artifacts**: in-container installs leave root-owned bind-mount dirs (`EACCES` on host) -- `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 from external access
- `profiles: [debug]`: services start only with `--profile debug`

## References

- `references/ci-testing.md` -- CI testing patterns for Docker images
- `references/dind-testing-patterns.md` -- Docker-in-Docker (DinD) testing patterns
- `references/bind-mount-ownership.md` -- root-owned bind-mount artifacts