rails 5.1 asset pipeline mgmt: yarn, webpack for all assets - css, fonts, images, ...

This details how best to manage assets in the rails pipeline for deployment to heroku:
http://www.akitaonrails.com/2017/06/28/rails-5-1-heroku-deployment-checklist-for-heroku

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Make sure you have 2 boot files, first the canonical Procfile to be used by Heroku in production:
1 web: bin/rails server -p $PORT -b 0.0.0.0

Second, a Procfile.dev to be used only in your development environment:

1 web: ./bin/rails server
2 webpacker: ./bin/webpack-dev-server

This is how you fire up the webpack server that will compile your assets in real-time during development. You need to also remember to run these two dependency commands first (before webpacker):

1 yarn install
2 bundle install
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First, to install webpack to an existing Rails 5.1 app, per:
https://github.com/rails/webpacker#installation

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Or add it to your Gemfile, run bundle and ./bin/rails webpacker:install or bundle exec rake webpacker:install (on rails version < 5.0):

# Gemfile
gem 'webpacker', '~> 2.0'

# OR if you prefer to use master
gem 'webpacker', git: 'https://github.com/rails/webpacker.git'
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***Remember that webpacker requires Yarn version >= 0.20.1. (Download and install the latest version from https://yarnpkg.com/lang/en/docs/install/)
Order of installs:
$ brew install yarn
and then the rake task:
$ bundle exec rake webpacker:install
The preceding yields many lines of log, ending with:
Webpacker successfully installed 🎉 🍰

To precompile assets:
bundle exec rake assets:precompile