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.
3 jobs for gitlab-runner-community in 1 minute and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #389
docker-alpine s390x
build-s390x

00:00:40

passed #386
docker-alpine x86_64
build-x86_64

00:00:25

failed #385
docker-alpine s390x
build-s390x

00:00:24