[motto]

I have been using shipping containers as storage units for many years and from experience have been made well aware of their shortcomings.
First up is the flat top inevitably has undulations which causes water to pool in the low areas and despite the corrosion resistance of the steel and protective coatings nature wins in the end.
Another problem is condensation inside the container. This depends on atmospherics and can be quite remarkable under certain conditions. This is exacerbated by placing damp or wet objects in the container then closing it up for a period of time.
My recommendation to anybody using a seatainer for storage is to install a roof over it. Even sheets of corrugated iron spaced up with packing boards to give it some air gap and fall will improve the interior environment. It must be secured of course.
A ventilator is also a good idea. Some people instal a whirligig.

David