Using a container for a chicken coop is a terrible idea. It's going to be roasting hot in the summer unless you cut holes for ventilation, which is going to be ten times as hard to do with a steel container as it would be with an ordinary shed. It will also be freezing cold in the winter. You want something smaller than a container so the chickens' body heat and the manure breaking down will keep the coop a few degrees warmer than the outside. Speaking of manure, you will want something with a dirt floor to let the ammonia leach out. The doors on a container aren't going to be very well suited for a chicken coop, and again they'll be much harder to modify than wooden doors would be. I'd check FB marketplace and Craigslist for a used shed or chicken coop if you're on a budget.