Add Dockerfile to repository

[AlexGustafsson]

The container image that is available on GHCR doesn't have its source defined in the repository. For transparency and for the build to be reproducible, please add the Dockerfile to the repository.

Please also add the correct OCI annotations so that automated tooling such as scanners can correctly identify the source repository of the image.

See https://github.com/opencontainers/image-spec/blob/main/annotations.md#pre-defined-annotation-keys.

Of special importance is the org.opencontainers.image.source annotation.