GitLab CI: Add npm install to new build stage

.gitlab-ci.yml

-image: alpine:latest
+image: docker pull mhart/alpine-node
+
+stages:
+  - build
+  - deploy
+
+npm:
+  stage: build
+  script:
+    - node --version
+    - npm --version
+    - npm install
 
 pages:
   stage: deploy