Okay, so I did some tests on my own, and it seems Ngrok can give you different destination each time. I’ve seen the remote endpoint sometimes up and sometimes down and having different address each time 
When it works it works, but when it’s not it can be a peer TCP closed, or it can be unreachable, or other errors. So for now I think relying on Ngrok is not a good idea
I’m assuming Ngrok is just for dev or for convenience anyway, not production setup right?
So I suggest to just rely on docker networking instead. What OS are you running? I just searched and it seems for Linux you need to enable the host.docker.internal first. See here: How to connect to the Docker host from inside a Docker container? | by Tim van Baarsen | Medium