### Purpose
The OCI image spec specifies well-defined
[annotations](https://github.com/opencontainers/image-spec/blob/main/annotations.md)
that can be added to images.
Theses annotations can then be used by other tools to gather more
information of an image.
This PR adds the `org.opencontainers.image.source` to allow tools such
as [renovate](https://github.com/renovatebot/renovate) to find the
release notes of a give version.
~~I've only done this change for `Dockerfile`. Should I also add the
label to the other dockerfiles?~~
I've now added the source annotations to all `Dockerfile`s & action
workflows.
### Testing
None, change was done by following the [renovate
documentation](https://docs.renovatebot.com/modules/datasource/docker/).
This adds builds for the discovery and relay servers in the same manner
as Syncthing, so that Docker images are kept up to date with releases.
Inspired by and closes#8834.
Co-authored-by: Migelo <miha@filetki.si>
