Set BUNDLE_FROZEN to true

To follow rubygems' security adisory
https://github.com/rubygems/rubygems.org/security/advisories/GHSA-hccv-rwq6-vh79:

.gitlab-ci.yml

@@ -9,6 +9,7 @@ include:
 
 variables:
   DOCKER_VERSION: "20.10.15"
+  BUNDLE_FROZEN: "true"
 
 workflow:
   rules: &workflow_rules