Dockerfile node_modules caching

[tulma95]

Docker build time could be improved by caching node_modules. This can be done by first copying only package.json and installing packages and then copying rest of the code.

More info: https://www.aptible.com/documentation/faq/deploy/dockerfile-caching/npm-dockerfile-caching.html
and
https://stackoverflow.com/questions/35616170/how-to-cache-node-modules-on-docker-build