Build image for applications using JRuby and yarn/NodeJS, e.g. Rails with webpacker.

Currently, the supported versions are:

• JRuby 9.1.17
• NodeJS 8.9.3
• Yarn 1.3.2

The JRuby version determines the version of the Docker image.

Basic example Dockerfile for a Rails-App using webpacker and JRuby in app/Dockerfile.