For most storage use: just level ground and corner cribbing (4x6 timbers under the corner castings). Concrete piers or a slab are nicer but not required. The container distributes its weight through the corner castings, not the bottom rails.
A shipping container's frame loads through eight corner castings — the steel boxes at each corner. As long as those eight points are level and supported, the container is stable. That's why a concrete slab is overkill for most storage applications: you only need support at the corners, not the full footprint.
The minimum: level ground (within ~3 inches of flat across the footprint) and 4x6 pressure-treated timbers (or concrete blocks, or railroad ties) under each corner casting. This keeps the bottom rails off the ground, prevents rot, and lets the container drain.
Better: gravel pad (crushed stone, 4-6 inches deep, compacted) under the entire container with timbers on top. The gravel handles drainage; the timbers handle load transfer. Best: concrete piers at each corner for permanent placement. Slabs only make sense for container homes or if you'll be driving heavy equipment through.
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