Update .gitlab-ci.yml

.gitlab-ci.yml

@@ -40,7 +40,7 @@ build backend:
     - /kaniko/executor 
       --context git://gitlab.labranet.jamk.fi/conduitmiau/conduit-backend/
       --dockerfile Dockerfile
-      --destination $CI_REGISTRY_IMAGE:backend:${CI_COMMIT_TAG}
+      --destination $CI_REGISTRY_IMAGE:backend_${CI_COMMIT_TAG}