Add Docker image for akf CLI

[HMAKT99]

Summary

A Docker image would make it easy to use AKF in CI/CD pipelines without installing Python.

What needs to happen

1. Create a Dockerfile in the repo root
2. Base on python:3.12-slim
3. Install akf from the local source
4. Set entrypoint to akf
5. Add usage examples to the Dockerfile or a README section

Example usage

```bash
docker run --rm -v $(pwd):/work ghcr.io/hmakt99/akf stamp /work/report.md
```

Acceptance criteria

• docker build . works
• docker run can stamp and inspect files
• Image size is reasonable (<200MB)