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>

.gitlab-ci.yml

 # Image from https://hub.docker.com/_/gcc/ based on Debian
-image: gcc
+image: gcc:9
 
 autotools:
   stage: build