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docs/containerization.md

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+# 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