Ask yourself where any moisture comes from. If the container is air-tight, what is the problem – once the humidity is controlled at a sufficiently low level? The kit car appears to be a fire risk in an enclosed space while it is fuelled.
A dehumidifier is the fairly obvious route – as long as you are not trying to dehumidify the whole atmosphere by emitting dry air and replacing it with damp air.
You only need to condition at most a couple of thousand cubic feet (~60 cubic metres) of air and keep the temperature above dew point as long as the doors are kept closed and ventilation closed.
That means insulation to reduce heat loss, a decent desiccant dehumidifier (compressor types are hopeless below about 10 Celsius) and sufficient heat to ward off the dew point. Continued surface treatments (oil and such-like) would be a good idea, too.
IF you are only worried about the lathe (not boxing in the whole container🙂 ), box it in completely with good insulation and warm it to above dew point. Some means of desiccation for the box might be needed.
I collect little more than a couple of litres a week, in my heavily insulated and ‘fairly air-tight’ workshop, per week. It is about 30 cubic metres. I expect the water is due to inevitable air changes and some moisture through the concrete floor – even though it has a membrane below the top 200mm-250mm there is about 9 metres of perimeter open to the exterior.
A decent dehumidifier has a short pay-back, if you set the cost against the potential damage to you possessions.