I don't think a standard shipping container's walls will stop much of anything that's ballistically energetic, but agree that a double layer of sand bags inside would do the trick for a threat environment where you're 99% concerned with small arms fire.
A vehicle ramming the wall composed of a CONEX with double sand bags inside would likely be sufficient to knock a hole in the container. I'd suggest that's not a show stopper, just that phase 2 of the fortification plan is an effective anti-vehicle ditch or other obstacles. If you're just worried about civilian wheeled vehicles it need not be too involved, certainly not on the same level as a true anti-tank ditch. Depending on your manpower involved in setting up the position and equipment available, you could probably have your trench put in concurrently with siting the containers, and use the fill from the trench to do your sand bags.
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