Apply the `latest` tag in a more consistent way

Recently it happened that two pipelines got started on the same branch at nearly the same time: this led to the latest tags being intermixed, some coming from the first pipeline and some coming from the second.

This definitely confused me at some point, and can be much more confusing for people who are less used to the fun and games of containers and our CI pipelines.

With this MR we should be able to guarantee that all the latest tags come from a single pipeline.