it can’t be a bag for dry food because it won’t keep the food dry.
What dry foods actually need to be in a watertight container and how often do you immerse your packaged dry food in water or leave it in a humid environment? Back when I hiked a lot our circle of friends packed all the dry food we took into reusable fabric bags and had no problem keeping it dry for weeks in snow and rain.
Quite a lot of dry food is packaged in paper or carton—flour, cereal, couscous, sugar etc. For some reason (at least where I live) most dry food that are my staples like rice, pasta, buckwheat is packaged in plastic but could just as well be packaged in paper. It's not like rice or pasta is more vulnerable to humidity than couscous or sugar (packaged in paper/carton).