Use Android Push Notifications in Local Area Network

So, we are building a solution which is containerised using docker-compose and we have a flutter app with it. We plan to use it both on the web as well as on a local server away from the internet. Are there any ways to set up notifications to the application without having to rely on an outside service ? Ie , a notification service that can be dockerized and run without an internet. The mobile app can shift between the main server and local server , so what way can we achieve this?