not sure, but i know that a lot of weapon storage places, like the museum at Parris Island, keep there storage at a constant 70 degrees, with a humidity of no more than 50 to 55%
If your set on a container, you will have to coat the inside, and install a suitable dehumidifier and ventilation system,
as far as gun-safes, I do golden rods and desiccant in everything including the ammo and reloading lockers. power goes out here from time to time, so when the rods arnt powered up, the desiccant takes over. humidity here is a constant battle. you must have a backup for your primary system at all times so to speak.
the pacific northwest, I would build a cover, roof, 4 walls on a slab, around the container,insulate between the floor and the container(concrete does condensate,even with a vapor barrier, so a good coat of thoraseal on top should do it) and the walls of the container, install your ventilation on top of the container, squirrel cage to move air, dehumidifier to dry air thus keeping heat out of box add a gaspack and chiller to control temp.(1 ton) now delving back to the tinfoil side, add your door in the cover shed, leave enough distance to open the installed vault door in the container. (keeps the rust off the vault door as well. I would give myself a 10’gap between the front of the container and the front of the structure. that way you could install a generator for those days that power is down due to storms, would auto start when power drops, keeping your collection dry.
slab should be 6"thick with 33"deep by 12" wide footers.High strength concrete(5800psi). thats what they made me pour when i built my shop, due to the fact we hit pure sand in one corner when digging the foundation. and ive got multiple ton machines in the building.
in a 24"X24’slab i have as much HS5800 concrete in the footers as i have in the 6"thick slab. you could park a tank on it. plus you have to add rebar as well.
I know. overkill.