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.