[CI] Re-add build tag for docker-bench-security

.gitlab-ci.yml

@@ -120,6 +120,7 @@ clair:
 # Run docker-bench-security and upload the results
 docker-bench-security:
   stage: security-tests
+  tags: [build]
   script:
     # Change the Docker Compose to use the "testing" image, not yet pushed on production registry
     - sed -i -e "s|$MODIFIED_IMAGE_FULL|$MODIFIED_IMAGE_FULL_TEST|g" $MODIFIED_IMAGE/docker-compose.yml