[gnome-runtime-images] ci: add basic workflow rules

[Jordan Petridis <jpetridis src gnome org>]

commit 9f1eb08d5eddd33714b6972eb3207d222c96e8fe
Author: Jordan Petridis <jordan centricular com>
Date:   Fri Jul 8 20:45:30 2022 +0300

    ci: add basic workflow rules
    
    This will elliminate 'merge-request' pipelines, which are not
    useful for our usecase

 .gitlab-ci.yml | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index bf44a3e..1bd0f2d 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,5 +1,10 @@
 image: quay.io/buildah/stable:v1.26
 
+workflow:
+  rules:
+    - if: '$CI_COMMIT_TAG'
+    - if: '$CI_COMMIT_BRANCH'
+
 # Buildah can't use 'overlay' driver when running inside docker
 variables:
     STORAGE_DRIVER: 'vfs'