[ProfoundMan]

Relatively bulletproof, and depending on the furnishings, darned near fireproof. Structurally strong enough to NOT collapse during earth tremors or earth movement, and an ultimate fortress in serious times of anarchy or civil unrest. And it is entirely modular.

This is a little misleading. There's an early Doomsday Prepper episode where a guy puts a hole in one with a 10/22. There's a Mythbusters episode where they prove as little as 4 inches of dirt on the top collapses the roof. The framework around the container is strong as that's what supports the stacking of them but the sides are really quite lightweight - by design. They are for shipping after all.

They are an affordable solution for many applications but they are not a fortress by any stretch. Just so everyone knows.

[Army Air Corps]

One could address the roof and wall issues that you mentioned and do so with modest effort.

[Kartographer]

As I remember no the .22 did not penetrate the cargo container.

I also found this from FR:

ISO containers are constructed of thin Corten type steel with corrugations to give strength. Typical container walls will stop a .22 Long Rifle, and perhaps weaker pistol bullets. Most rifle bullets will easily penetrate the sheet metal of a container. Even the corner posts are just 1/4 inch thick square or rectangular steel tubing, which a decent rifle will penetrate.

While I agree the walls aren't bullet proof you have to agree that they would beat the hell out of a sheet of dry wall! ;-)