ci: use buildah/podman instead of docker
well, docker has licensing issues, and the version shipped in Fedora is getting quite old now. We have a free open source alternative through buildah/podman/skopeo.
Build our building image in the CI too, so updates can be achieved by just triggering the bootstrap job.
There are still a few things to fix, so please do not merge yet.
Edited by Benjamin Tissoires