Reefers can be set to whatever temperature they need to be at. Here's what a major container line has to say about that: https://www2.nykline.com/liner/cargo_advisory/chill_cargo.ht...

Bottom-line, 13 degrees centigrade is what you're aiming for along with 50 m3/h of airflow (50 cfm in US units.) Airflow because bananas ripen in the presence of ethylene and ripening bananas release ethylene. That's the reason putting bananas in a paper bag ripens them faster.