Improve Gobra Package Information

[ArquintL]

Recently upgrading the docker/build-push-action action changed how published Gobra images appear (before, after).
This seems to be related to multi-architecture Docker images and also manifests as an additional architecture called unknown/unknown (cf. this issue).

While the immediate workaround is fairly simple, i.e., setting provenance: false for this action, I took the opportunity to add several other meta-information about our images according to the OpenContainers Annotation Spec. While all labels appear in the manifest (JSON), only the description is extracted and separately rendered by GitHub, as shown here (note that this image has been published for testing purposes, we in general do not publish images on branches).

Additionally, we use several tags per image, i.e., the branch name, (short) commit hash, and latest for the master branch.