[ticket/12775] Add a container_factory class and remove functions_container

https://tracker.phpbb.com/browse/PHPBB3-12775
https://tracker.phpbb.com/browse/PHPBB3-12420
https://tracker.phpbb.com/browse/PHPBB3-12774

PHPBB3-12775
This makes sure we create a new container on the filesystem for each possible phpbb_root_path. Is this still required with the new code?