Although I have trouble reproducing it I still experience weird test failures whenever I create or updated separate MRs in a short amount of time. These failures are usually coming from unexpected display settings, although each job is running within docker in an isolated environment.
I don't have any explanation for this, and I have confirmed that separate X server are spawned. However, the prefix used for the tests is initialized to /home/gitlab/.wine and I suspect this could perhaps be mounted and shared across containers? It could also explain the failures, if, for instance wineserver starts while another instance (invisible as isolated in a separate container) is running and has updated the registry on disk with different display settings while the tests are being executed.
This is hypothetical but nonetheless I think it's better to keep the prefix in a job workspace sub-directory.