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.
3 jobs for gitlab-ci in 57 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10135
docker
autotools

00:00:57

passed #10137
docker
doc

00:00:24

passed #10136
docker
makefile

00:00:23