Dockerfile: use gitlab-runner from community

gitlab-runner has been moved to community, so we need to obtain it from
there. As we were getting gitlab-runner from edge anyway, switch the
image to use edge.
2 jobs for gitlab-runner-community in 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #383
s390x docker-alpine
build-s390x

00:00:35

passed #384
x86_64 docker-alpine
build-x86_64

00:00:23

 
Name Stage Failure
failed
build-s390x Build
No job log