I recall reading long long ago of hippie efforts to build geodesic domes from the roofs of old cars, and it was pointed out then, that much of the cost of a house is in the foundation, piping, electrical, heating/AC, windows, etc., so if you come up with a free "frame", you don't save that much.
And I've read of people wanting to repurpose old school buses, which are cheap enough. But one problem is, they're generally not insulated, not tall enough, just not made to use as RVs, etc.
With the containers, I will point out that if they need to be recoated, with a regular abrasive blast and heavy duty coatings, that is expensive. So factor that into the building and maintenance cost.
Assuming you are not using these right next to the shipyard they come from, you'll run into a pretty good chunk of change hauling them and unloading/placing them as well.
You may have a lot of strength in these things, but trying to show that if you don't have mechanical properties, material certificates, etc., may be a challenge.