add npm cache to CI

.gitlab-ci.yml

@@ -7,8 +7,12 @@ build site:
   stage: build
   script:
     - cd app
-    - npm install --progress=false
+    - npm ci --cache .npm --prefer-offline
     - npm run build
+  cache:
+    key: ${CI_COMMIT_REF_SLUG}
+    paths:
+      - app/.npm
   artifacts:
     paths:
       - app/dist