Shipping containers are a bad idea because 1) you need to cut holes for doors/windows and that destroys the stack ability, 2) any shipping container old enough to not be resellable (and therefore costly) is not in a good condition to make a house out of, 3) you still need a foundation and pipes/power so you're not saving that much money compared to a wood frame, and 4) steel sucks at insulation like you said (not to mention shipping containers don't have eaves).