I think you can do it with a html-redirection. The code of the site you have to safe as /var/www/html/ should look like the following:
<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<meta http-equiv="refresh" content="0; url=https://cloud.mydomain.com/nextcloud">
<script type="text/javascript">
window.location.href = "https://cloud.mydomain.com/nextcloud"
</script>
<title>Page Redirection</title>
</head>
<body>
<!-- Note: don't tell people to `click` the link, just tell them that it is a link. -->
If you are not redirected automatically, follow this <a href='https://cloud.mydomain.com/nextcloud'>link to https://cloud.mydomain.com/nextcloud</a>.
</body>
</html>
Name of the file has to be index.html