gitlab-ci.yml: Allow tagging images built on upstream branches

If we're on the gstreamer namespace, we should push all images built on that namespace to the registry. This is needed to, f.ex., update the docker image used in stable builds, which is needed for: cerbero!367 (merged)

@alatiera @tpm: does this sound ok to you? AFAICT we won't use any newly-tagged images unless we also change the *_IMAGE values in gitlab/ci_template.yml, so this should always be safe. Only problem I can see is that pushing the 'latest' tag can be argued to be semantically not-quite-correct.