It is completely standard in the shipping industry to have lopsided routes, where one direction has 100% full containers, and the return direction a much lower percentage, like 50%. The shipping cost of different routes takes this into account.
Sounds absurd, but that's how it is.
In my case it's actually cheaper and much more flexible to build it as you would a traditional building. I live 300 miles from the nearest port, so that doesn't help. I also looked into the container sized temporary buildings used on construction site, and they are made from scratch (not from old shipping containers).