A 20-footer weighs about 2 tonnes and a 40-footer weighs about 4 tonnes. These are well within a lot of load ranges for a derrick. You can build a big A-frame and get a trailer under it. You'll need chains, winches and cargo handling knowledge.
Now, trailers tend to have been left on nice flat ground but of course people are going to want them elsewhere. At this point you'll want as you said a crane. However by 2000ad they're everywhere (my players are looking for one now) and craning stuff around is bread-and-butter work for most people by the time 2000ad rolls around.
By the way, those containers use Weathering steel (often known as "Cor-Ten" which is a trademark). While this stuff rusts slowly your player-made welds will rust at a normal rate unless they use specialist equipment which really isn't worth the bother. The walls are generally just under 2mm thick of mild steel equivalent.
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