Would a shipping container be considered a building for flood purposes? I would think not since it wouldn't be affixed to a permanent site.
Building definition from NFIP:
• A structure with two or more outside rigid walls and a fully secured roof, that is affixed to a permanent site; or
• A manufactured home (a “manufactured home,” also known as a mobile home, is a structure: built on a permanent chassis, transported to its site in one or more sections, and affixed to a permanent foundation); or
• A travel trailer without wheels, built on a chassis and affixed to a permanent foundation, that is regulated under the community’s floodplain management and building ordinances or laws.
“Building” does not mean a gas or liquid storage tank or a recreational vehicle, park trailer, or other similar vehicle, except as described above.