title: Webpack environment variable are unknown argument
body: <p>I'm trying to create a webpack setup but can't manage to make it work with the environment variables.
  I have read the doc more than 10 times but can't find the solution.</p>
  
  <p>My package json has this script:</p>
  
  <pre><code>"build": "webpack --config webpack.config.js --env.MODE=production "
  </code></pre>
  
  <p>But when I'm running it, node is telling me:</p>
  
  <pre><code>webpack: Unknown argument: --env.MODE=production
  Usage: webpack --config &lt;path to webpack configuration file&gt; e.g. ./webpack.config.js
  Description: Provide path to a webpack configuration file
  Documentation: https://webpack.js.org/configuration/
  </code></pre>
  
  <p>EDIT: I finally found the solution.
  The webpack-ci installed version was "webpack-cli": "^4.0.0-beta.2".
  I downgraded it and it's now working fine.</p>