Generalize config (env var) configuration, move NEXTCLOUD_TRUSTED_DOMAINS out of entrypoint.sh

Folks;

It is an inconvenience to have to remove / reinstall nextcloud to change NEXTCLOUD_TRUSTED_DOMAINS via environment variables.
If extra config is done by altering config/config.php, changes are lost on re-install and must be done manually, each update.

Suggest that setting NEXTCLOUD_TRUSTED_DOMAINS be moved out of entrypoint.sh (only set @nc update/install) and moved to (new) file config/extra_config.php which sets extra config (at nextcloud start) similarly to how email is configured by config/smtp.config.php

If all possible config settings (eg: overwritehost, overwritewebroot) have corresponding $ENV vars (NEXTCLOUD_<var name, uppercase>) for config/extra_config.php,, all config can be done by environment variables, in a generalized manner.

Env vars with space separated entries translate to array (eg: trusted_domains), else 'var' > 'value', in config/extra_config.php.

I noted this inconvenience (manual intervention required post nextcloud install/update) while porting / generalizing docker/nextcloud as part of www.rossco.org endeavors.

Regards;
Bill

Start by reading entrypoint.sh and the existing config/smtp.config.php pattern, then compare how NEXTCLOUD_TRUSTED_DOMAINS is currently handled. Define the scope for config/extra_config.php and the supported NEXTCLOUD_ environment variables, including space-separated arrays. Done means configuration survives reinstall or update and the proposed environment-based settings work consistently.