Should I protect a shipping container from direct contact with the ground?
Containers are designed to be supported in 4 corners and span the rest.  

If your container is longer than 40 feet (12 metres), you will find 8 support points, not 4.  The most important ones are the middle ones that are 40' (12m) apart.  

Left in contact with the ground, they will rust uncontrollably, with rate wildly depending on the soil type, pH and wetness.  

The contact with the earth will also be a haven for critters.   what kind of critter will depend on the height, so you get to decide whether the underspace is fit for scorpions, cats or horses!  Choose a height that will attract the least dangerous or most desirable critters.