It seems that burying shipping containers was a thing for a while, and maybe it still is. A shipping container is not made to be buried, nor to have heavy weight on top of it. Is it a thing because it might be a fast way to get an underground place? How long before it would be completely rusted out and collapse? The size is limiting. The treated lumber flooring is a health issue.
If I was going to have something underground, I would want it to be poured and reinforced concrete, not cinder block where moisture can weep through the joints. Then I would want steel beams across the roof and more reinforced concrete. Yes, more work and money, but anything worth having is worth a little quality.
Me? I would not call it a root cellar nor a bunker. I would call it a storm cellar. How many people live in areas where tornadoes happen? Build the storm cellar, then outfit it for food storage or as a bunker. Build shelves for storage, but keep them covered so your preps are not seen immediately. Call it a bunker, and you have gotten people's attention in other ways.
My grandparents had a root cellar in town that they never used, but had filled in. There were tornadoes occasionally in the area, but I never knew anyone who had a storm cellar. Also, I wouldn't put it in a place where anyone and everyone driving by would see it. I might put it out back, close to the main house, and even have a short tunnel from the basement going there, with steel doors at both ends of the tunnel for reinforcement from storms and break ins.