title: Symfony 4 - Webpack Encore bootstrap css not in output
body: <p>I am trying to introduce Webpack Encore into my Symfony 4 app to manage the JS &amp; CSS assets.</p>
  
  <p>I installed <code>yarn</code> and <code>nodejs</code>.</p>
  
  <p>Then <code>composer require encore</code> and then <code>yarn install</code>.</p>
  
  <p>I have the <code>app.js</code> file in <code>assets/js/</code>.</p>
  
  <pre><code>/*
   * Welcome to your app's main JavaScript file!
   *
   * We recommend including the built version of this JavaScript file
   * (and its CSS file) in your base layout (base.html.twig).
   */
  
  // any CSS you require will output into a single css file (app.css in this case)
  require('bootstrap');
  require('../css/app.css');
  
  // Need jQuery? Install it with "yarn add jquery", then uncomment to require it.
  const $ = require('jquery');
  
  console.log('Hello Webpack Encore! Edit me in assets/js/app.js');
  </code></pre>
  
  <p>Installed bootstrap and other dependencies with yarn:</p>
  
  <pre><code>{
      "devDependencies": {
          "@symfony/webpack-encore": "^0.22.0",
          "bootstrap": "^4.3.1",
          "jquery": "^3.3.1",
          "popper.js": "^1.14.7",
          "webpack-notifier": "^1.6.0"
      },
      "license": "UNLICENSED",
      "private": true,
      "scripts": {
          "dev-server": "encore dev-server",
          "dev": "encore dev",
          "watch": "encore dev --watch",
          "build": "encore production --progress"
      }
  }
  </code></pre>
  
  <p>Then run <code>yarn encore dev --watch</code></p>
  
  <p><strong>Problem</strong></p>
  
  <p>I do not get any bootstrap css in my <code>public/build/app.css</code>.</p>
  
  <p>Do I need anything more than <code>require('bootstrap');</code> in <code>webpack.config.js</code>?</p>