$ mkdir rails5_1_0_beta1-test
$ cd rails5_1_0_beta1-test
$ touch Gemfile

source 'https://rubygems.org'

gem 'rails', '5.1.0.beta1'

$ bundle install --path vendor/bundle
$ bundle exec rails new . --webpack=react
       exist
      create  README.md
      create  Rakefile
      create  config.ru
      create  .gitignore
    conflict  Gemfile
Overwrite /Users/yuki/Src/framework/rails/rails5_1_0_beta-test2/Gemfile? (enter "h" for help) [Ynaqdh] y
       force  Gemfile
         run  git init from "."

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      create  package.json
      remove  config/initializers/cors.rb
         run  bundle install
Fetching https://github.com/rails/sass-rails.git
Fetching https://github.com/rails/webpacker.git
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching gem metadata from https://rubygems.org/.........
Fetching version metadata from https://rubygems.org/..
Fetching dependency metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "sprockets":
  In snapshot (Gemfile.lock):
    sprockets (= 3.7.1)

  In Gemfile:
    sass-rails was resolved to 6.0.0.beta1, which depends on
      sprockets (~> 4.x)

    sass-rails was resolved to 6.0.0.beta1, which depends on
      sprockets-rails (< 4.0) was resolved to 3.2.0, which depends on
        sprockets (>= 3.0.0)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.
       rails  webpacker:install
The git source https://github.com/rails/sass-rails.git is not yet checked out. Please run `bundle install` before trying to start your application
       rails  webpacker:install:react
The git source https://github.com/rails/sass-rails.git is not yet checked out. Please run `bundle install` before trying to start your application

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$ bundle update
Fetching https://github.com/rails/sass-rails.git
Fetching https://github.com/rails/webpacker.git
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$ bin/rails webpacker:install
      create  app/javascript
      create  app/javascript/packs/application.js
       exist  bin
      create  bin/webpack-dev-server
      create  bin/webpack-watcher
      create  bin/webpack
   identical  bin/yarn
      create  config/webpack
      create  config/webpack/development.js
      create  config/webpack/production.js
      create  config/webpack/shared.js
      append  .gitignore
         run  ./bin/yarn add --dev webpack webpack-merge webpack-dev-server path-complete-extname babel-loader babel-core babel-preset-latest coffee-loader coffee-script rails-erb-loader glob from "."
yarn add v0.20.3

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$ bin/rails webpacker:install:react

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// Run this example by adding <%= javascript_pack_tag 'hello_react' %> to the head of your layout file,
// like app/views/layouts/application.html.erb. All it does is render <div>Hello React</div> at the bottom
// of the page.

import React from 'react'
import ReactDOM from 'react-dom'

class Hello extends React.Component {
  render() {
    return <div>Hello {this.props.name}!</div>
  }
}

document.addEventListener('DOMContentLoaded', (e) => {
  ReactDOM.render(
    <Hello name="React" />,
    document.body.appendChild(document.createElement('div'))
  )
})

$ tree config/webpack
config/webpack
├── development.js
├── production.js
└── shared.js