Steel is cold, can conduct heat very easily and creates thermal bridges if you don’t insulate with the proper materials. If you look around, you can find many sites showing multiple ways of insulation. From rockwool, to styrofoam, many options are shown. The key-word with insulation is condensation. Steel will condensate when left unprotected. With rockwool or styrofoam type of insulation, you will always have a cavity somewhere, where the steel is open to the air. On those spots, it will condensate given the right conditions causing issues later on. In my opinion, the only proper insulation is sprayfoam insulation. This type of insulation is sprayed on the walls, roof and other parts of the container after which it will expand immediately. It will fill all holes and create an airtight layer of insulation. If applied correct, there will be no thermal bridges or condensation spots. The Irish container house featured in an earlier post also applied sprayfoam insulation. https://sprayfoamuk.com/case-studies/grand-designs-container
Inside or outside?
So where do you need to add the insulation? Inside, outside or both? Because there is always moisture in a house, I would always apply insulation on the inside, but only a thin layer. A couple of centimetres, because you don’t want to loose too much inside space. The largest part would need to be added on the outside. My suggestion would be to add the underlying construction for the cladding first, apply the sprayfoam, remove the excess and cover it with a vapor barrier before you add the outer cladding. Check the video below how fast sprayfoam can be applied.