Use the gcc:9 image for gitlab-ci builds.
Specify an explicit version of the base container image for more
repeatable results. Note that this will need to be bumped periodically.
Signed-off-by: 
Marvin Scholz <epirat07@gmail.com>
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Specify an explicit version of the base container image for more
repeatable results. Note that this will need to be bumped periodically.
Signed-off-by: Marvin Scholz <epirat07@gmail.com>