title: process.env.API_URL is undefined
body: <p>I built a project using React at the front and Strapi (based on Node.js) at the back.</p>
  
  <p>On the client side, I'm trying to access my api url like so:</p>
  
  <pre><code>const apiUrl = process.env.API_URL || 'http://localhost:1337'
  const strapi = new Strapi(apiUrl);
  </code></pre>
  
  <p>But process.env.API_URL is undefined. If I log process.env all I get is an object containing:</p>
  
  <pre><code>NODE_ENV: "development"
  PUBLIC_URL: ""
  </code></pre>
  
  <p>How can I access my api_url ?
  I'm guessing there is a file where I should define API_URL myself?</p>