Add NEXTCLOUD_TRUSTED_DOMAINS environment variable

See merge request !2

This is needed for newer Nextcloud versions, supposedly from v15 on.

Also see:

- https://help.nextcloud.com/t/solved-referrer-policy-and-content-security-policy-broken-on-nginx-only-setup/49212/9

This is also reported on /settings/admin/overview if not present.

just to be sure.

thanks @lalop !