Make sure you are totally recovered by the time you get locked up. Try to get a job in the kitchen ASAP so you can eat as much as you want. The foods on comissary are typically garbage except for canned meats (which are better than nothing even though they are loaded with sodium. Start slow with your diet and lifting (assuming they have weights where you'll be), and gradually increase your cals and your intensity. Getting in the kitchen is your only hope bro, unless you just want to stay tone and lean. Smash your core a lot as it will help you get strength back when you get out if you weren't able to lift much while you were in. Been there bro.......good luck!