Move step of single image test to the release MR

In !3098 (merged), we changed the release task to update the GitLab version in the Docker single image test job.

While this is a good addition, writing .gitlab/ci/docker-images.gitlab-ci.yml will only happen in stable branches, and never in main.

Since we need this to be in main, we should move this step out of the task and in the monthly release MR steps.