nginx: Allow extending config with overrides

To facilitate this, the default redirect at / can be disabled, even if
the default remains at redirecting to the webmailer.

The extensions are within the host scope and are read from
$ROOT/overrides/nginx/*.conf.

docs/compose/docker-compose.yml

@@ -29,6 +29,7 @@ services:
     - "$BIND_ADDRESS6:587:587"
     volumes:
       - "$ROOT/certs:/certs"
+      - "$ROOT/overrides/nginx:/overrides"
 
   redis:
     image: redis:alpine