"Truck Driver" is not a meaningful description of McLean when he "invented" the precursor of the modern shipping container. At that point in his career he owned a very large trucking company.
As specifically noted in TFA, he had the idea at a time when he was a truck driver. He did not get to implement it until much later, at which time he owned a very large trucking company. It is not a coincidence that he needed to own the very large trucking company to be equipped to implement his idea.