ci: Use commit ref slug instead of branch name

as default container tag name for uploading, because branch names can
includes characters that aren't valid for tags (such as '/').

Signed-off-by: Andreas Hartmann <hartan@7x.de>

.gitlab-ci.yml

@@ -36,7 +36,7 @@ build:
   script:
     # N/B: There could be more than one merge-request open with this branch's HEAD
     - |
-        IMAGE_TAG="${CI_COMMIT_BRANCH}";
+        IMAGE_TAG="${CI_COMMIT_REF_SLUG}";
         if [[ -n "$CI_COMMIT_TAG" ]]; then
             IMAGE_TAG="${CI_COMMIT_TAG}";
         elif [[ -n "$CI_OPEN_MERGE_REQUESTS" ]]; then