GitLab CI runner for Debian unstable is .. very unstable
On my fork, nearly every job using debian:unstable
results in a big red box "Something went wrong on our end." at the top, and the job log disappears, and it may be marked as pass or fail.
The "Raw" log button available on the right hand allows you to see what actually happened, including whether it was a success or not.
This is definitely an upstream bug, but it appears to only be a problem with the Web UI.
However, many of the failures are due to Debian package fetch problems, and this happens often enough to be annoying, and we probably want to avoid Debian images for a while.