gitlab-ci: Run jobs on the gcc:9 image.
Use a versioned gcc container image for more consistent test results. This is the same version we're using for other projects, but of course it will need to be bumped periodically. The current gcc release is 10.2. The oldest supported release is 8.4, so this is in the middle of the support window. Signed-off-by: Marvin Scholz <firstname.lastname@example.org>