Build Docker images in Framagit's CI and push them from there

Docker hub Autobuild isn't available anymore for free tiers. While providing an open-source free tier, it requires submitting a form with manual proofs every year, which is annoying.

@tcit has told me it's possible to actually build the Docker images in our CI, and then push them to the Docker hub from CI.

https://docs.gitlab.com/ee/ci/docker/using_kaniko.html Mobilizon example