Update .gitlab-ci.yml

.gitlab-ci.yml

 image: node:12
 
-cache:
+cache: &global_cache
   key: ${CI_COMMIT_SHA}_node_modules
   untracked: true
   paths:
@@ -17,6 +17,8 @@ install_dependencies:
   stage: prepare
   script:
     - yarn install 
+  cache:
+    <<: *global_cache
 
 .build_job:
   stage: build