gitlab-ci.yml: enable builds for git tags

Set rule to run our build pipeline for git tags as well.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

.ostc-ci/gitlab-ci.yml

@@ -107,3 +107,5 @@ update-docs:
         - meta-*/**/*
         # Run this job in case the pipeline changes.
         - .ostc-ci/*.yml
+    # Run the build when a tag is placed.
+    - if: '$CI_COMMIT_TAG'