Question
No, level firm ground works.
You do not need a concrete slab for a shipping container if the ground is level and firm. Gravel, paving slabs, or compacted hardcore work well. A 20ft container puts about 500 kg of weight on each corner casting. Wooden blocks or steel plates under the corners spread the load and prevent sinking on soft ground.
Did you know?
A concrete slab is not always required because containers are designed to transfer loads through the corners. In many situations, properly prepared corner supports are more important than a full slab. 
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