I'll help you to elaborate a Dockerfile that works fine and stable, also in swarm environments (next step will be OpenShift and upgrades).
It would be helpful to add logging information when socket.io rejects a connection, i.e. log the detected vs the configured origin.
@SimonQ I am not using the last version of this plugin (I made a fork when the regex issue arised) so I could not reproduce your problem. So I just tried on a testing version of the forum I am in charge and I observe the same behaviour. So it is not just you. I suppose it is a question of tweaking the css though. I will try to have look but I am VERY far to be an expert.