chore: add OCI label indicating source

[mikaello]

This label is documented as:

org.opencontainers.image.source URL to get source code for building the image (string)

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

This label can be used by tools to give a link back to this repository, e.g. renovate will use this label when it creates pull request to update this docker image.