gitlab-ci: automatically run the CI on pushes to `ci/*` branches

Last commit limited the CI to master and MRs, but to avoid having to
manually trigger CI runs, let's add a 3rd, automatic way: by pushing to
a branch named `ci/*` (or `ci-*` or just `ci`) (which you can delete
afterwards, the pipeline results will remain).

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>

.gitlab-ci.yml

@@ -66,6 +66,7 @@ containers:ubuntu:
   only:
     - master
     - merge_requests
+    - /^ci([-/].*)?$/
   artifacts:
     when: on_failure
     untracked: true