Update .gitlab-ci.yml

.gitlab-ci.yml

-image: node:10
-
+image: node:latest
+# This folder is cached between builds
+# http://docs.gitlab.com/ce/ci/yaml/README.html#cache
+cache:
+  paths:
+    - node_modules/
 pages:
   script:
     - cd website
-  - yarn
-  - yarn run build
+    - npm install
+    - ./node_modules/.bin/gatsby build --prefix-paths
   artifacts:
     paths:
       - public