Finish self-hosted GitHub runner

[matthiasgoergens]

#499 introduced a prototype self hosted GitHub runners. But a few tasks remain:

• Document how and especially where the runners are set up, so that anyone can learn enough to go and fix them. How do people get access to the runner?
• Undo the downgrade to Ubuntu 22
• Use the official images instead of copy-and-pasting from some random website.

In case I need to spell it out:

Some advantages of using the official images include:

• We get (security) updates.
• We have less code to maintain and test.
• Someone wanting to learn how the system works, so they can work on it, can just read the official documentation (and/or ask other people in the wider world who also use the official images), instead of having to reverse engineer the code we copy-and-pasted from some random website.

We already do far too much copy-and-paste coding in the rest of our code base.