As per current version 0.15+ you have at least 3 options for feeding prisoners. You can:
- Have a Nutrient paste dispenser in your prison (prisoners feed themselves)
- Have a warden assigned (warden feeds each prisoner individually - brings food)
- Just have a stock of food on the ground in your prison (prisoners feed themselves)
The nutrient pase dispenser is the best way. Just place it in your prison wall, have the hopper outside of the prison so the colonist doesn't need to go in. If you have a lot of prisoners you can even have 2-3 hoppers. Now you need to turn off the "Gets food" option in your prisoners tab (so warden doesn't bring them food).
I also recommend setting the dispenser's food for something you have in excess (so you are not wasting some excellent meat on your prisoners.)
OT: I also have a stock of tribal wear in my prison because I strip them from all of their nice clothes.