Will we be able to create any sort of shareable cargo containers beyond backpacks, like ammo crates, medical supplies, or boxes with food and drinks?

Not for 3.15. You can't make crates. But the intention is that you will eventually be able to buy boxes that you can then put stuff in and other people can share. And the first time we would be able to do that would be the cargo refactor, where you'll be able to buy an empty box and use that in your cargo to store things. And then other people will be able to come and interact with that box.