There is no easy way to make the internal mysql server public without changing the code and I don't know what that will break.
I think you can setup a ssh tunnel between your minecraft server and the Cloudron.
Something like https://www.linode.com/docs/databases/mysql/create-an-ssh-tunnel-for-mysql-remote-access/
Do this on Cloudron:
docker inspect mysql | grep IPAddress
"IPAddress": "172.18.0.4",
So, 172.18.0.4 is the IP of mysql server. Then, you can get the db credentials using the web terminal of the LAMP app:
env | grep CLOUDRON_MYSQL_
You can then connect using the above credentials as:
mysql --host=172.18.0.4 --user=<username> --password=<password> <db>
After the above works, you just setup SSH tunnel accordingly.