Test against `nightly` omnibus image

With the upgrade to %15.7 and particularly the password related change caused some trouble on the release date.

This is way too late and we should catch these kinds of updates much earlier.

There a multiple ways how to address this, though the easiest is probably to add two jobs to our pipelines - one for CE and one for EE - with the nightly gitlab-omnibus images from docker hub.

We can also have a scheduled pipeline which may run somewhen after these nightly images are deployed - I suggest that these nightly jobs are allowed to fail, but at least warn us about potential upcoming breaking changes for us (most likely in the test suite).