Zippered freezer bags, one for each portion. They are designed to take hot water, since frozen food is blanched before freezing. Note that by the time you take your water off the stove or fire and pour it into the freezer bag, it's below boiling. Freezer bags are not, however, designed to be immersed in boiling water, and that shouldn't be necessary. There are other types of bags designed for boil-in-bag situations.

Of course you can rehydrate your food in your cooking pot, if you don't mind washing the pot afterwards. (Unless you put the pot back on the fire, the food won't stick to it to any great degree.) If you're traveling solo, you can eat ouf ot the pot, a bit less messy than trying to eat out of a freezer bag. I prefer to eat out of the freezer bag and use the rest of the hot water in the pot for tea, which I drink from the pot. YMMV, here!

Note that freeze-drying, unlike dehydrating, is not something you can do at home--it requires specialized equipment.