gitlab-ci: build with clang and do multiple builds in one test-step

Also, let one docker image do multiple builds. We fetch a fedora docker
image, and then install 250 MB of packages. That alone takes a lot of
time and resources. Instead of running a large number of docker images
that only do one build, let one image do several builds.

Also, install ccache. Hopefully this way we can benefit from
building the same sources multiple times.

Also note that building docs does not work currently with clang,
due to g-ir-scanner. See commit 05568860.
4 jobs for th/gitlab-ci-clang in 38 minutes and 41 seconds
Status Job ID Name Coverage
  Test
passed #100476
allowed to fail
checkpatch

00:05:49

passed #100477
f28_build

00:38:41

passed #100478
f_latest_build

00:37:17

passed #100479
allowed to fail
f_rawhide_build

00:28:20