Enhance CI pipeline deploy step

With #2 (closed), we get deployments to Docker Hub. However, we currently have to duplicate the rules > changes for each deploy job as variables are not exposed to the changes block.

This is currently blocked by https://gitlab.com/gitlab-org/gitlab/-/issues/8177 and thus by https://gitlab.com/gitlab-org/gitlab-runner/-/issues/1809, but seems to be resolved soon.