title: Gatsby sitemap.xml and rss feed using wrong domain name
body: <p>I'm using Ghost with gatsby-starter-ghost and running into an issue with the generated sitemap.xml from gatsby-plugin-advanced-sitemap as well as the RSS feed URL.</p>
  <p>My domain name is <a href="https://www.officehomelife.com/" rel="nofollow noreferrer">https://www.officehomelife.com/</a> and is hosted on <a href="https://officehomelife.netlify.app/" rel="nofollow noreferrer">https://officehomelife.netlify.app/</a></p>
  <p>The problem is <a href="https://www.officehomelife.com/sitemap.xml" rel="nofollow noreferrer">https://www.officehomelife.com/sitemap.xml</a> is generating URLs based off of the netlify domain name and not my domain name. It is also using the netlify domain name for the RSS feed <a href="https://feedly.com/i/subscription/feed/https://officehomelife.netlify.app/rss/" rel="nofollow noreferrer">https://feedly.com/i/subscription/feed/https://officehomelife.netlify.app/rss/</a></p>
  <p>I believe this value should be coming from gatsby-starter-ghost\src\utils\siteConfig.js where I have it defined:</p>
  <pre><code>    siteUrl: `https://www.officehomelife.com`, // Site domain. Do not include a trailing slash!
  
      postsPerPage: 12, // Number of posts shown on paginated pages (changes this requires sometimes to delete the cache)
  
      siteTitleMeta: `Office Home Life`, // This allows an alternative site title for meta data for pages.
      siteDescriptionMeta: `The essential guide to working from home`, // This allows an alternative site description for meta data for pages.
  
      shareImageWidth: 1000, // Change to the width of your default share image
      shareImageHeight: 523, // Change to the height of your default share image
  
      shortTitle: `Office`, // Used for App manifest e.g. Mobile Home Screen
      siteIcon: `favicon.png`, // Logo in /static dir used for SEO, RSS, and App manifest
      backgroundColor: `#e9e9e9`, // Used for Offline Manifest
      themeColor: `#15171A`, // Used for Offline Manifest
  }
  
  </code></pre>
  <p>I tried deleting a .cache folder but that did not help, any other ideas?</p>
  <p>Thanks</p>