Have you actually talked to anyone in the cargo ship industry about your idea?
I worked in the container shipping industry. It doesn't sound like this person ever talked to anyone in the industry. It's not like "add a sail" hasn't been thought off loads of times before. I've seen a few times where people spend 6 months to run a project where they never talked to a person with experience. Then that project would eventually fail. Circumstances and restrictions sometimes change over time, so trying to solve the same thing every few years is a good thing. But "sail in a container", ehh..
If the sail is pulling, the container below would see a remaining uneven load from above. Is there a limit to this? Loading/unloading an unbalaned container sounds like a problem, but is it, while sitting on the ship?
A small to medium vessel fully loaded is like 150.000 tons. 10% of that is 15000 tons, imagine the force that needs to be transferred to the structure of the ship in a place where it was not designed to handle that force.
The typical problem with containers is non-standard or unsecured cargo pressing against the side walls, which are indeed very weak.