Draft: Apply the MR changes in the single Dockerfile tests

In !3289 (merged), we started testing the single Docker images by checking out the branch defined in the GITLAB_VERSION variable. While this solved the issue described in #1284 (closed), it introduced another issue: if we change single.Dockerfile in a merge request, the changes of the MR are not tested, since we're checking out another branch!

Since the changes we mostly want to test are in single.Dockerfile, we can fetch this file from the MR and build against that.

This would essentially fix what Evan stumbled upon in !3519 (comment 1266149551).

Closes #1383 (moved)