CI: add missing IS_DEFAULT_BRANCH variable

.gitlab-ci.yml

@@ -35,6 +35,7 @@ update-docker-image:
   script:
 
 # push to our registry
+    - if [ "${CI_COMMIT_REF_NAME:-}" = "${CI_DEFAULT_BRANCH}" ]; then IS_DEFAULT_BRANCH=1; fi
     - |
         /kaniko/executor \
           --context ${CI_PROJECT_DIR}/docker/code-analysis \