The container is named upon the phpbb_root_path, which is parsed to "translate" some specials characters. The semi-colon isn't part of them and crash the include when the phpbb_root_path contains one.
In order to avoid further exceptions to exclude, a single & unique identifier (as a hash of the path) could be used to avoid both conflict and manual exceptions (Credits to Nicofuma as discussed here https://www.phpbb.com/community/viewtopic.php?f=461&t=2318786&p=14093751#p14093751)