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.
2 jobs for gitlab-ci in 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10141
docker
autoconf

00:00:53

passed #10142
docker
cmake

00:00:24