Cache node_modules per Node.js version.

The node:latest image changes version every once in a while;
the explicit version key ensures that npm packages are up to date.

Author: RubenVerborgh

.gitlab-ci.yml

@@ -4,6 +4,7 @@ before_script:
   - npm install
 
 cache:
+  key: "$CI_PROJECT_PATH/$CI_BUILD_REF_NAME/$NODE_VERSION"
   paths:
   - node_modules/