ci: use cache instead of artifacts for node modules

.gitlab-ci.yml

@@ -1,5 +1,13 @@
 image: registry.gitlab.com/openstapps/app
 
+cache:
+  key: ${CI_COMMIT_REF_SLUG}
+  paths:
+    - node_modules
+
+before_script:
+  - npm install
+
 stages:
   - setup
   - build
@@ -20,6 +28,8 @@ setup:
     - docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN registry.gitlab.com
     - docker build -t registry.gitlab.com/openstapps/app .
     - docker push registry.gitlab.com/openstapps/app
+  cache: {} # disable irrelevant cache for this job
+  before_script: [] # do not run irrelevant before script for this job
   tags:
     - docker
 
@@ -30,7 +40,6 @@ build:
     - npm run build
   artifacts:
     paths:
-      - node_modules
       - www
 
 unit: