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/.env

@@ -97,6 +97,9 @@ COMPRESSION_LEVEL=
 # Web settings
 ###################################
 
+# Path to redirect / to
+WEB_DEFAULT=/webmail
+
 # Path to the admin interface if enabled
 WEB_ADMIN=/admin