gitlab-ci: Use the gcc:9 base image.

Use a versioned base image for jobs to reduce variance.

This version number will need to be bumped periodically.

Signed-off-by: Marvin Scholz <epirat07@gmail.com>
2 jobs for gitlab-ci in 1 minute and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #10279
docker
autoconf

00:01:53

passed #10280
docker
cmake

00:00:36