Position is the first thing, under a tree, in a shadow, prevailing breeze.
Orientation is next, the east sun is hotter than the west sun so try and hold down as much square footage as possible. The weakest sun light comes from the north, this being the best direction for doors or windows.
Heat goes to cold. So you need large ventilation openings in the top of your container/building. Power fans work great here. Storage inside the container has to allow for air movement otherwise it becomes insulation/temperature conductor and heat sink.
The outside of your container needs to have non electric conducting material creating a void of several inches to allow for air movement up the sides of your container. The roof should have a pitch of at least 6/12 starting a foot above the walls on one side the other side start 2 feet above the wall and continue it at a 6/12 pitch so it is above the other roof. The first roof will be smaller so it should be sunward if possible. You can cover this exterior with duct wrap, foil faced with the jacketed side out to reflect heat or use reflective metal.
Then put a tent over the whole thing, large vents in the top. The sides should touch the ground, but you should be able to raise and lower them allowing the cool or shaded side to let air in. Keep the sun sides closed.
This will allow for storage between the tent wall and the container. If it really gets hot you can put a sprinkler lawn hose in this void.