Have you already spoken with whatever concrete company you are using for the container piers? It's very possible they have a minimum order amount and your four piers might be less than the minimum anyway. If that is the case doing some extra piers for a freestanding deck wouldn't be a issue, but instead a solution. You can also consider throwing some scrap together in small enough to move squares for stepping stones. If you have to buy the concrete, might as well use it all.
A deck would be the way I would go, rather than having your eventual exterior siding at grade. In fact having your container home up a little higher even could be beneficial as well, being easier to get to any utilities run up through your floor. Something will go wrong eventually, and having access is always beneficial. Even if the benefit is just being able to find someone willing to go under there to fix it.
It sure is frustrating trying to figure out the whole binder full of things at once.
