Just to follow up on the other reply: it’s a little more expensive, but look for single-trip containers. These will have less rust / damage. Some retailers also advertise whether or not a given container is still water-tight (most single trip containers are).

For a chicken coop you likely don’t need a full 40’ box. The next standard size down is a 20’ box but you can also get down to 10’ pretty easily. This can help save on costs and planning.

You’ll still need a level support for the container to sit on. The most common method is leveling blocks for the corners to sit on. Plenty of videos on YouTube showing various methods for container placement / leveling.