Check Makefile for which ports to export and where to mount a volume. Mouting volume is not required. But you probably want it on different parititon.
OpenSSH in the transmission container will load .ssh/authorized_keys from that volume and allow you chrooted sftp access.
Also Transmission will load .config/settings.json which is Transsmision daemon settings. You can have there something like:
{
"rpc-whitelist-enabled": false
,"rpc-username": "user"
,"rpc-password": "password"
,"rpc-authentication-required": true
,"watch-dir-enabled": true
,"watch-dir": "/mnt/storage/watch"
,"trash-original-torrent-files": true
,"download-dir": "/mnt/storage/downloads"
}For more settings check https://trac.transmissionbt.com/wiki/EditConfigFiles