A bit off topic: any advice about integrating Webpack with Rails? Looks like the most popular gem to do that has only 19 stars on github... https://github.com/danott/webpack_rails
Your best bet is no integration at all. Just set up webpack to read from ./app/assets and output to ./public. Run it separately, preferable with a foreman Procfile for development (I've used Procfile.dev before).