# Containerization

Package your app and its environment together so it runs identically on every machine — dev, CI, and production.

## Tools

**Docker** is the standard. Everything else (Podman, containerd) is compatible with Docker images.

## Key Concepts

- **Image** — a snapshot of your app + OS + dependencies
- **Container** — a running instance of an image
- **Dockerfile** — the recipe for building an image
- **Local CI pattern** — run your test suite inside a container before pushing; catches environment drift before it reaches remote CI

## Minimal Dockerfile (Python)

```dockerfile
FROM python:3.12-slim

WORKDIR /app
COPY pyproject.toml .
RUN pip install uv && uv pip install --system -e ".[dev]"

COPY . .
CMD ["python", "-m", "mypackage"]
```

## Quickstart

```bash
# build
docker build -t myapp .

# run
docker run --rm myapp

# local CI — run tests inside the container
docker run --rm myapp pytest

# interactive shell for debugging
docker run --rm -it myapp bash
```

## Tips

- Use `.dockerignore` to exclude `__pycache__`, `.git`, `.venv`, `*.pyc`
- Pin the base image tag (`python:3.12-slim`, not `python:latest`)
- Layer ordering matters: copy dependency files before source so the install layer is cached