Ruby apps now get Yarn installed when the Webpacker gem is present
Change effective on 16 March 2017
Rails 5.1 ships with the ability to use webpack to manage your assets. This requires that the system compiling your assets has Yarn installed. When the Ruby buildpack detects the webpacker gem, it will auto-install Yarn on the system.
If you wish to have more control over the version of Node and Yarn installed you can manually specify the Node and Yarn versions you wish to have in your app. You can also read more about Yarn on our blog.
If you are trying to deploy the Rails 5.1.0.beta1 to Heroku, you will hit a bug in rails that will prevent your application from booting. This bug has been fixed in master, but not released at this time. If your app is failing to boot, you can specify this command in your Procfile:
web: bin/rails server -p $PORT -b 0.0.0.0
Alternatively you can directly specify Puma in your Procfile, and specify 0.0.0.0 for a host.