ci/container_builds: Only perform the container-build stage if needed

I mentioned something about this at GUADEC this morning and I think we can easily avoid doing the CI-rebuild step in most of the cases using this

Only try to rebuild container images in case the relevant files changed or if is triggered manually or via an env variable.

This way we can avoid even starting such useless jobs unless a relevant variable has changed.

Not sure if you want also make it always when .gitlab-ci.yaml changed, but it doens't seem needed to me.