That’s probably the issue. It likely expects the domain without the port. (mine doesn’t have the port)
I’m not using a docker dev install, so I really don’t know.
In my case (Windows WSL), the URL is http://localhost:4200, and to start Discourse I use this command: DISCOURSE_HOSTNAME=localhost UNICORN_LISTENER=localhost:3000 bin/ember-cli -u
So I’m thinking that maybe you need to define DISCOURSE_HOSTNAME or similar?