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 <epirat07@gmail.com>
1 job for gitlab-gcc in 1 minute and 32 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #10715
docker
build

00:01:32