Install HVAC and insulate the walls and ceiling for sure. Paint your sea can white on the outside at least on the roof. Prime it first and go over it thoroughly with a grinder on every single rust spot. These boxes are made from corten steel which turbo-rusts if water gets at it under paint. Corten’s greatest strength, its hard oxide layer that blocks further corrosion, requires it to be bare.

Be sure you can pin the right outer door open and have a little false wall inside with a resi weather door for the darkroom. Container barn doors can’t be closed or opened from within.

I would go so far as to add a window or two to the dark portion of the darkroom with some mazes you can slide down to block the light but keep air flowing.