Plain-old shellac is also food grade material. If you ever see the word "confectioner's glaze" on candy, or anything like that, or you see a slick coating on the outside, like Junior Mints, there's a good chance that it's shellac.
The question is whether or not there is any issue with heat - if there is, I would go with poly - if not, shellac. Mineral oil would be OK, too, as long as you're aware that you're going to have to oil the canisters from time to time.