gitlab-ci: Apply the "latest" tag only on success

Always tag built images as `test-$COMMIT` so that we keep a bunch of
those around in case a rollback is needed, using a GitLab Container
Registry cleanup policy to keep storage under control.

https://docs.gitlab.com/ee/user/packages/container_registry/#cleanup-policy



We can then apply the `latest` tag to officially publish the built
images only after the tests have passed successfully.

This allows projects to set up a cleanup policy tailored to their needs
and to easily retain images that are needed for long term support, for
instance the ones used to on release builds.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>