The syncthing docker has been updated to include the discovery and relay servers. They are disabled by default to ensure the syncthing docker remains a standard syncthing instance as before. To enable them, see the environment variables under the advanced section.
While this docker should auto-update just fine, you will probably need to delete it and re-install it in order to make use of the new options from the template. Just make a note (or screenshot) of the "Update container" page with your current settings, delete the container, then re-install it with your same settings/mount mappings in addition to any use of the new features. Syncthing will continue to work as before as long as it has access to the same config folder and data locations.