How to configure trusted domains depends on the deployment method. For the container env vars are required for host based setups, changes need to be added to the config.php file. (All) error messages and hints in ownCloud (this does not only affect the trusted domains message) assume a host based deployment, as ownCloud Core itself is of course not aware of the used deployment method.
Not sure if you really want to go that path, but that’s up to the engineering team.