Add Yarn caching

.gitlab-ci.yml

@@ -12,12 +12,13 @@ build:
   stage: build
   script:
     - cd app/
-    - yarn install
+    - yarn install --cache-folder .yarn
     - yarn build
     - mv ./build ../public
   cache:
     paths:
       - app/node_modules/
+      - app/.yarn
   artifacts:
     paths:
       - public