Metal sheds tend to make poor coops because they get oven-hot and are pathologically underventilated. However metal roofing and siding can be used perfectly well for a coop, because since YOU are the one building the coop, you can ensure that it has mucho plenty ventilation, and the stud structure (as opposed to how metal yard sheds are made) allows you to easily insulate the building.
And you almost certainly WILL need to insulate it -- in cold-winter climates, bare metal loses a lot of heat and (worse!) becomes a condensation/frost factory that prevents good ventilation and humidifies your coop; in hot-summer climates, unless you have a semi-open coop you may want to insulate at least the ceiling and S wall (if any) to reduce the roasting capacity of the coop. Just scrounge up some old panelling or thin plywood or suchlike to cover the insulation so chickens can't eat it, which otherwise they will.
Good luck, have fun,
Pat