Docker

# Docker Skill

> OS-level virtualization to deliver software in packages called containers.

## Ecosystem Graph

```mermaid
graph LR
  docker["Docker"]
  docker -- "deploys to" --> kubernetes
  docker -- "integrates with" --> github-actions
```

## Quick Start
Docker packages your application and its dependencies into an immutable image, ensuring 'it works on my machine' scales to production.

```bash
docker build -t my-app .
docker run -p 8080:8080 my-app
```

## Production Patterns
### Multi-Stage Builds
Never ship build tools (like compilers or dev dependencies) in your final production image. Use a `build` stage to compile your code, then copy only the compiled artifacts into a distroless or alpine base image for the final stage.

## Architecture & Scaling
### Image Caching
Docker builds images in layers. Order your `Dockerfile` from least frequently changed (OS dependencies, package managers) to most frequently changed (source code). This ensures Docker caches the heavy steps and dramatically speeds up CI pipelines.

## Error Recovery
Use a process manager like PM2 or tini as `PID 1` inside your container to properly handle OS signals (SIGTERM/SIGINT) and reap zombie processes, allowing graceful shutdowns.

## Security Notes
Never run your container as the `root` user. Explicitly create and switch to a non-root `USER` at the end of your Dockerfile. Scan your images using tools like Trivy or Docker Scout to catch CVEs before deployment.

## Relationships
**Works Well With**: [github-actions](/skills/github-actions)

**Deploys To**: [kubernetes](/skills/kubernetes)

## References
- [Docker Docs](https://docs.docker.com/)

## Why use this skill
Use this when your agent works with **docker** — structured patterns beat pasted docs and prevent common hallucinations.

## AI pitfalls
- Referencing CLI flags or config keys that do not exist
- Using outdated major versions of tools
- Skipping lockfile or version pinning in examples

## Production checklist
- [ ] Secrets in environment variables, not source code
- [ ] Error handling and logging in place
- [ ] Rate limits and timeouts configured

## Related skills
- [`kubernetes`](../kubernetes/SKILL.md) — deploys to
- [`github-actions`](../github-actions/SKILL.md) — integrates with

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> **Last Verified:** 2026-07-02