Add GitLab CI config

.gitlab-ci.yml

+image: node:latest
+
+before_script:
+  - 'curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -'
+  - 'echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list'
+  - apt-get update && apt-get install -y yarn
+  - yarn install
+
+build:
+  script:
+    - npm run build
+  artifacts:
+    paths:
+      - public/packs
+      - public/assets
+    expire_in: 1 week
+