Synology NAS,, docker image, and untrusted domain

I am working to get NextCloud operating on a SYNOLOGY FS2419. I have been successful in installing the official docker image and accessing the app within my network both through a browser and via the windows app.

I had hoped to be able to leverage Synology’s function and internal router to then access via the web. When I try this I get the message “Access through untrusted domain. Contact your administrator, edit the trusted domains setting…”

My question - In a docker based implementation where will I find the config.php file?