FYI not Docker but I’m using Ember CLI with a non-docker dev setup in the cloud
All I had to do was redirect calls to port 4200 (in my case using nginx) which connects to the ember server which in turn connects to the Rails server on 3000 (no change there)
This suggests for docker you probably just need to run the container with a port mapping to send incoming calls to 4200 to get it to work with ember CLI?