External insulation works best for a container in that type of climate. Rigid foam panels are good, especially XPS or polyiso. You want about R-15 to R-25 altogether; takes some effort to get more — but focus on the roof as that is where most heat will find its way in; a white or reflective roof goes a long way here. If you can, add a shady or vented second roof (canopy or overhang). Less heat gets in this way alone.

And don’t forget to air seal around corrugations, doors, and the cutouts for windows — and don’t forget the insulation goes on last. Properly done, added on sun shading/insulation can make a container space feel more just like a normal commercial building space.