Currently config.php is included in the following locations (3.1.0-b2 line numbers):

• phpBB\common.php:22
• phpBB\includes\functions_container.php:33
• includes\functions_container.php:52
• phpBB\phpbb\di\extension\config.php:37

All of which (following the backtrace) come from common.php's container creation (https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/common.php#L94). The necessary variables from config.php should be passed in/down instead of including the file 4 times and then only the necessary inclusion in common.php occurs.