Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -59,8 +59,8 @@ docker-build:
   stage: build
   image: docker:git
   script:
-    - docker build -t $DOCKER_NAMESPACE/gateway:$TAG .
-    - docker save -o build/image.tar $DOCKER_NAMESPACE/gateway:$TAG
+    - docker build -t $DOCKER_NAMESPACE/gateway:${CI_COMMIT_TAG:latest} .
+    - docker save -o build/image.tar $DOCKER_NAMESPACE/gateway:${CI_COMMIT_TAG:latest}
   artifacts:
     paths:
       - build/image.tar