Add image source docker label so tools can generate changelog for images

[halkeye]

Tools like renovate and dependabot depend on docker container labels to generate changelogs

Eg. https://docs.renovatebot.com/modules/datasource/docker/

Ideally org.opencontainers.image.revision should be injected as well, but I don't really know where the CI scripts are

[tulir]

Probably not going to add any labels before figuring out how to deduplicate dockerfiles

[halkeye]

I only just realised what this meant. I can deduce the docker files I think. Do you have a recommendation on which repo to submit a pr too?

[tulir]

Probably needs to be in mautrix/ci