For a number of years a 40-foot Seacan, spray foamed and wood-floored, was my "office" in northern Manitoba...where -30 or -40 and sometimes even colder is definitely a "thing". Standard pedestrian doors cut and installed into each end. A couple of space heaters inside would get that thing far beyond comfortable inside, to the point where they would need to be turned down quite a bit if the doors were not opened frequently.
I love the idea of this container as described in the OP, but...most guys are talking about it as though it's a room that they think they will kick back and relax in. It's not; anybody who actually gets one likely has way more toys than the paltry few shown in the original posted pic. By the time you add insulation and then some kind of rack system on at least one side to hold your toys, there's not a lot of space to stretch out in. Guns on both sides? They're likely arranged vertically, with grips and stocks and other pointy things sticking into the centre of the "room". No realistic space for chairs or anything else, really.
Inside height 72 inches. Subtract a couple of inches at both top and bottom for insulation, flooring, etc. and most people can't even stand up straight inside there. Not a nice place to spend time.
A mannequin? Fat chance.
The original pic shows a cute little workbench near the door on the right side. But...just eyeballing the scale of the pic and keeping in mind the dimensions...that bench is, what, maybe 15 inches deep? Have you ever tried to do any work on a 15-inch-deep bench, lol? It's just a waste of the tiny amount of space you have to work with in there; I'd bet folding money that a guy who purchased the unit as shown would literally never choose to use that work bench as anything other than a storage shelf...until he finally decided to rip it out altogether.
I can see vertical gun storage along one side, with some combination of vertical racks and shallow shelves on the other side as desired, and maybe some larger shelves at the back end...maybe...
This is not a small, comfy-cozy room in which to sit and play with or work on your guns. It's a medium-sized walk-in closet for safe storage of them, allowing pretty nice, easy access to any individual item without needing to move a dozen others. I'm not being negative here, I love the idea, but expectations need to be realistic.
edited to add: Oops...just saw how old this thread is. Any updates on this idea?