title: Webpacker creates only the manifest.json file, so assets are missing
body: <p>I'm building a Rails 5.2 application with the webpacker gem.</p>
  
  <p>I followed the installation instructions so that I have the <code>app/javascript/packs/application.js</code> file, and the <code>&lt;%= javascript_pack_tag 'application' %&gt;</code> in my layout.</p>
  
  <p>After having started my unicorn server, the <code>bin/webpack-dev-server</code> process and I'm accessing the app, the network tab of the webbrowser shows me that he can't download the the <code>/packs/application-bdddc5e750615fa56298.js</code> file from my server due to an End of file error from the webpack server (500 error).</p>
  
  <p>Doing an <code>ls -al</code> in the <code>public/packs</code> folder show me the following:</p>
  
  <pre><code>drwxr-xr-x  2 zedtux zedtux 4096 Apr  4 16:35 .
  drwxr-xr-x 16 zedtux root   4096 Apr  4 16:35 ..
  -rw-r--r--  1 zedtux zedtux  352 Apr  4 16:35 manifest.json
  </code></pre>
  
  <p>And the <code>manifest.json</code> contains :</p>
  
  <pre><code>{
    "application.js": "/packs/application-bdddc5e750615fa56298.js",
    "application.js.map": "/packs/js/application-45e23109c9e9d11521f0.hot-update.js.map",
    "entrypoints": {
      "application": {
        "js": [
          "/packs/application-bdddc5e750615fa56298.js"
        ],
        "js.map": [
          "/packs/application-bdddc5e750615fa56298.js.map",
          "/packs/js/application-45e23109c9e9d11521f0.hot-update.js.map"
        ]
      }
    }
  }
  </code></pre>
  
  <p>Here is my <code>config/webpacker.yml</code> file :</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_root_path: public
    public_output_path: packs
    cache_path: tmp/cache/webpacker
    check_yarn_integrity: false
    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
  
    # Extract and emit a css file
    extract_css: false
  
    static_assets_extensions:
      - .jpg
      - .jpeg
      - .png
      - .gif
      - .tiff
      - .ico
      - .svg
      - .eot
      - .otf
      - .ttf
      - .woff
      - .woff2
  
    extensions:
      - .mjs
      - .js
      - .sass
      - .scss
      - .css
      - .module.sass
      - .module.scss
      - .module.css
      - .png
      - .svg
      - .gif
      - .jpeg
      - .jpg
  
  local:
    &lt;&lt;: *default
    compile: true
  
    # Verifies that versions and hashed value of the package contents in the project's package.json
    check_yarn_integrity: true
  
    # Reference: https://webpack.js.org/configuration/dev-server/
    dev_server:
      https:
        key: |
          -----BEGIN PRIVATE KEY-----
          ...
          -----END PRIVATE KEY-----
        cert: |
          -----BEGIN CERTIFICATE-----
          ...
          -----END CERTIFICATE-----
        cacert: |
          -----BEGIN CERTIFICATE-----
          ...
          -----END CERTIFICATE-----
      host: 0.0.0.0
      port: 3035
      public: 0.0.0.0:3035
      hmr: true
      # 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
  
    # Extract and emit a css file
    extract_css: true
  
    # Cache manifest.json for performance
    cache_manifest: true
  </code></pre>
  
  <p><em>This app is using local as the development environment, so I renamed the development key to local</em></p>
  
  <p>BTW I have enabled <code>hmr</code>, but even with it disabled I have the same result.</p>
  
  <p>Why are the files not written in the folder? And how could I fix this?</p>
  
  <h2>Update</h2>
  
  <p>When I stop the <code>bin/webpack-dev-server</code> process and I reload the page, webpack recompile and the files are loading correctly in the webbrowser.</p>
  
  <h2>Update 2</h2>
  
  <p>Opening the pack URL in a new tab show me the 500 error. I noticed that the pack URL is missing the 3035 port. Adding it returned me the expected JavaScript code.</p>
  
  <p>So how to fix it so that the pack URL includes the server port?</p>