http://localhost/joplin won't work because the localhost of the client and the host of the server are not the same.
If you haven''t tried it yet, I'd put a http:// in front of the IP in the docker-compose file.
Do you use a reverse proxy to direct the traffic to docker with port 22300? I tried this kind of local setup once without a reverse proxy and set the base url to http://192.168.x.x:22300, which worked fine...