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.
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