Managed to use the openSUSE build service (obs) to build a first project (a container) https://build.opensuse.org/project/show/home:jornfranke. I followed this tutorial: https://en.opensuse.org/openSUSE:Build_Service_Tutorial (it is about packages, but looking at existing examples I managed to build a container).
You can use it to build own distributions: https://en.wikipedia.org/wiki/Open_Build_Service
An impressive and very interesting service. Glad that I learned it a bit and happy to dive deeper - a very useful skill.