Merge #21619: ci: Run self-hosted ci

<pre>Merge #21619: ci: Run self-hosted ci fa41a917356a7f5e0f3286b227ced7f2e6797e67 ci: Run self-hosted ci (MarcoFalke) fa52a40f0ebb7aa33af210a18e525eec7bb6887f ci: Make cirrus cache folders relative to cirrus base dir (MarcoFalke) fa278412a023ac150da764a465567f2970348449 ci: Restart docker before run (MarcoFalke) fad4f48e0763d111369656dea9575a789d2016e8 ci: [refactor] Create setting for ephemeral config in .cirrus.yml (MarcoFalke) Pull request description: Due to our heavy use of the Cirrus CI community cluster, some tasks may take a long time to get scheduled. While it is possible to use "Compute Credits" to get immediately scheduled on the cluster, I couldn't find a sponsor that'd be willing to cover the total cost, if all tasks were paid for with credits. However, it is also possible to bring our own runners to Cirrus CI. For testing purposes, a single task will be transformed to run on the DrahtBot infrastructure. If all goes well, the other tasks can be moved, too. ACKs for top commit: hebasto: ACK fa41a917356a7f5e0f3286b227ced7f2e6797e67, I have reviewed the code and it looks OK. Tree-SHA512: 513738daec36da8cd48a8f11e687ff0b7dfaba1ae4ed2fa77e7b043f88fd52bf5c0dbad2768e13df88518323917f08348cb62be6376a423142921f8d2c59a938</pre>