Supposedly John, containers are sealed units, so the only time you should have problems with dampness is if you let it in, via vents.
If it was me, I would get a couple of dozen silica gel bags (about 1kg each) and hang half of them up inside, near the walls, but not in contact, and have say a couple of months rota, where you swap the bags out for regenerated ones.
We used to have a 20ft container for storing bulk fabric outside at a company where I worked, and the boss had already done that to it before I joined, and we never had any mould or condensation issues with it for the 5 years or so I was there. The spare bags were hung on the radiators in the office block to regenerate them, so they were always ready for swapping over.
Just a suggestion for any easy way out.
John