Livebook attached node in docker

I am trying to connect livebook to a running phoenix app to do some BI analysis. Following these fly.io guides
Connecting Livebook to Your App in Production
Setting a Static Cookie for Elixir

I have my phoenix app and livebook both as containers and using docker-compose to launch. Both are behind a reverse proxy with a load balancer between the app and proxy.

the local IP for phoenix app is 10.0.3.22 running on an internal port 4000 but when I try the attach node mode and enter < containre_name > : < container local ip > with the cookie value it shows node unreachable.

Livebook is being accessed from the web but it would need local connection ip as the connection is internal ?

You probably only have the port 4000 exposed from the docker container, which will not work.
You could read about epmd and how to setup erlang / elixir without epmd to only expose one extra port to make it work.