I was unable to find a coherent explanation for running multiple Drupal 8 websites from a single codebase in a UNIX environment. Here are some notes I hope will be helpful.
1. Rename "examples.sites.php" to "sites.php". Edit sites.php to list your individual domain names as shown below. You can have an unlimited number of domains drawing on the resources of a single codebase. Make sure to remove the * before each line.
* The following examples look for a site configuration in sites/example.com:
* URL: http://localhost:8080/example
* $sites['8080.localhost.example'] = 'example.com';
$sites['domainname1.com'] = 'domainname1.com';
$sites['domainname2.com'] = 'domainname2.com';
$sites['domainname3.com'] = 'domainname3.com';
$sites['domainname4.com'] = 'domainname4.com';
/// REPEAT AS NECESSARY ///
* URL: https://www.drupal.org:8080/mysite/test/
* $sites['8080.www.drupal.org.mysite.test'] = 'example.com';
* @see default.settings.php
* @see \Drupal\Core\DrupalKernel::getSitePath()
* @see https://www.drupal.org/documentation/install/multi-site
2. Go to root, and in your "sites" folder you will need to create individual folders that correspond to each of the domainnames you enumerated in sites.php. Each of these folders will contain an unedited settings.php file that the server can write to as it sets up your site.
3. If you prefer to set up your file structure in root, please see this article.