title: Webpacker error when running server for new Rails app
body: <p>Just created a brand new Rails app and receiving this error when I run the server:</p>
  <pre><code>Webpacker can't find application.js in /public/packs/manifest.json. Possible causes:
  1. You want to set webpacker.yml value of compile to true for your environment
     unless you are using the `webpack -w` or the webpack-dev-server.
  2. webpack has not yet re-run to reflect updates.
  3. You have misconfigured Webpacker's config/webpacker.yml file.
  4. Your webpack configuration is not creating a manifest.
  Your manifest contains:
  {
  }
  </code></pre>
  <p>Here's my <code>webpacker.yml</code>:</p>
  <pre><code># Note: You must restart bin/webpack-dev-server for changes to take effect
  
  default: &amp;default
    source_path: app/javascript
    source_entry_path: packs
    public_output_path: packs
    cache_path: tmp/cache/webpacker
    webpack_compile_output: false
  
    # Additional paths webpack should lookup modules
    # ['app/assets', 'engine/foo/app/assets']
    resolved_paths: []
  
    # Reload manifest.json on all requests so we reload latest compiled packs
    cache_manifest: false
  
    extensions:
      - .js
      - .sass
      - .scss
      - .css
      - .module.sass
      - .module.scss
      - .module.css
      - .png
      - .svg
      - .gif
      - .jpeg
      - .jpg
  
  development:
    &lt;&lt;: *default
    compile: true
  
    # Reference: https://webpack.js.org/configuration/dev-server/
    dev_server:
      https: false
      host: localhost
      port: 3035
      public: localhost:3035
      hmr: false
      # Inline should be set to true if using HMR
      inline: true
      overlay: true
      compress: true
      disable_host_check: true
      use_local_ip: false
      quiet: false
      headers:
        'Access-Control-Allow-Origin': '*'
      watch_options:
        ignored: '**/node_modules/**'
  
  
  test:
    &lt;&lt;: *default
    compile: true
  
    # Compile test packs to a separate directory
    public_output_path: packs-test
  
  production:
    &lt;&lt;: *default
  
    # Production depends on precompilation of packs prior to booting for performance.
    compile: false
  
    # Cache manifest.json for performance
    cache_manifest: true
  </code></pre>
  <p>After running <code>./bin/webpack</code>, it recommended I install webpack-cli via <code>yarn add -D</code>.
  After I did this, I got the following error:</p>
  <pre><code>Error: Cannot find module 'webpack-cli'
  Require stack:
  - ../node_modules/webpack/bin/webpack.js
      at Function.Module._resolveFilename (internal/modules/cjs/loader.js:1030:15)
      at Function.Module._load (internal/modules/cjs/loader.js:899:27)
      at Module.require (internal/modules/cjs/loader.js:1090:19)
      at require (internal/modules/cjs/helpers.js:75:18)
      at ../node_modules/webpack/bin/webpack.js:143:5
      at processTicksAndRejections (internal/process/task_queues.js:93:5) {
    code: 'MODULE_NOT_FOUND',
    requireStack: [
   '../node_modules/webpack/bin/webpack.js'
    ]
  </code></pre>
  <p>I don't have a <code>manifest.json</code> - shouldn't this have been created automatically?</p>
  <p>Any ideas?</p>