if you build out "fur out" the inside with 2x6 you could insulate with batts like any home. Of course this reduces the interior space all around by nearly 1'.
2x4 furring would still allow for a thinner batt (insul) but possibly a lower rate of insul.
im not totally up to knowledge on the R ratings of various spray foam insulation a but generally they allow for the least furring. You could get way with 1-1/2" furring and spray the spaces between. Spray foam is more expensive but is used for the purpose of minimizing any extra framing and where spaces don't allow for the volume and or installation of regular batt insulation.
obviously, like any building there will be some heat transfer through the wood framing.
perhaps reviewing a cross action detail of a steel classes was assembly would help you.
if you are heating the unit I would make sure that you install a poly barrier on the warm side to prevent condensation build up on the inside of the steel wall.
But as I said, review a detail and a small amount of research for wall assemblies in our climate and your gold.
the unit I fitted was in New Zealand so in this instance there was no need for insulation.
Edit
i should clarify: the furring is easy. You could use wood or steel stud. Simply attach a stud(s) along the perimiter of the floor and ceiling and infill at 16" or 24" on centre.
Since the wall will not be structural you can get away with 24" spacing.