fix: Fix Relay usage for users of ALLOWED_HOSTS (#17900)

Forward the Host header so ALLOWED_HOSTS actually works (and does not
require the user to add "sentry" explicitly)

This still requires the user to add `host.docker.internal` explicitly,
which we can unlikely fix.

config/reverse_proxy/nginx.conf

@@ -63,5 +63,7 @@ http {
         location / {
            proxy_pass http://sentry;
         }
+
+        proxy_set_header Host $host;
     }
 }