Trusted proxy if using dynamic DNS and the proxy accesses php via Unix domain socket?

I am running php fastcgi over a Unix domain socket with caddy as the web server doing TLS. This works all really nicely, except the admin page of nextcloud telling me my trusted proxy setting is not right. Well, there is the problem: caddy is using the external ip number of the computer as the proxy up address in the headers, which changes every once in a while as it is a home dsl behind a nat router. If I would access php via a tcp socket, I would probably have my local private ip address as the proxy address, but php should only use the Unix domain socket, nothing else.

So how do I tell nextcloud that the proxy is fine and there can be no bad proxies because it is accessible only via Unix domain socket?