My 20' weigh 5000lbs empty. We just moved them with a cane. Both weighed 8500 lbs with stuff in them.
Mine aren't on poured footings we just used precast solid concrete blocks. This was done for legal reasons, those being they're not permanent structures, therefore not subject to any building code or other rules or regulations. This is due to the fact that our land is a part of a wildlife trust, and rules of the trust determine what 'structures' are allow on the property.
You should be able to fabricate a clamp after putting something like 5/8" or so threaded rod in the footing. Bend a hook in the end inserted in the wet concrete. This is how my new building is attached to the foundation so it should work fine for a container