GitLab 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.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>