gitlab-ci: Use versioned gcc:9 image for builds.

Use a versioned gcc container image as the base for more
repeatable results. The version will need to be bumped
periodically.

Also reformat to follow current recommendations by moving
the contianer image and tags under the `default` key.
Signed-off-by: Marvin Scholz's avatarMarvin Scholz <epirat07@gmail.com>
parent 475e92a1
Pipeline #2102 passed with stage
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