Every one always says XMPP and there were a lot of recommendations for ejabberd. I tried this recently and it was a total disaster, I do not have a working chat server. If I followed the docker instructions the server would just crash with no details of what went wrong. Where it should have been creating a default server config file it was instead creating a directory with the wrong permissions then promptly crashing. I tried following their documentation but after about 6 hours of messing about and adding more and more I still couldn’t get a client to login to it. I have no idea how to make this work.

So whatever the solution ultimately is I can’t recommend Ejabberd.