This is completely wrong, nothing is using that port...
Are you sure that you've forwarded the correct port with the correct protocol, UDP in this case and yes, you only need 8211 UDP.
Please don't forward other ports! It's not necessary, I only list ports which are necessary!
Are you sure that your friends have the correct public IP which is also reflected in the log from the container? If it times out then it is not reachable from outside.
What is the exact message? Do they see a loading screen or not? If they see a loading screen for about 30 seconds then chances are high that the public IP in your config is not correct or that the port forwarding is not working properly.
BTW if you have a domain name assigned to that IP for example yourdomain.com then they simply can connect with yourdomain.com:8211