Cache generated by `update-gems-cache` cannot be retrieved by jobs that use the `gitlab-org-docker` runners manager

We can see that https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/1030489591 (with the gitlab-org-docker tag) doesn't download any cache, while the cache is generated in https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/1030421903 (with the gitlab-org tag).

I think we'll need to introduce a new update-gems-cache-for-docker-jobs that runs with extends: .docker_job (which adds the gitlab-org-docker tag) so that other jobs with this tag can take advantage of the distributed cache.

Ideally, we wouldn't need this if we'd use Kaniko to build the Docker images, allowing to use the gitlab-org tag instead of the gitlab-org-docker tag.