CI: WIP: attempt to clean up the registry before leaving

CI: WIP: attempt to clean up the registry before leaving

According to multiple sources, referenced in
https://engineering.facile.it/blog/eng/continuous-deployment-from-gitlab-ci-to-k8s-using-docker-in-docker/

The garbage collector of the registry won't clean up docker images that
still have blob references. We should clean up the manifests instead
of simply overwriting the tag.

Note: this requires to set up a personal token with api access from the
maintainers in the form of (for instance): "PERSONAL_TOKEN_bentiss"

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>