I am a huge fan of Use-What-You-Got!
Knowing roughly where you live, I would think that would be rock. We have an abundance of it in Maine, especially Downeast.
It is hard saying as I am not there, and while positioning would be key, could you situate it just right so that the long part of your container was on the uphill side, sitting on flat rocks, but then as the ground sloped away, you built a rock wall to support the other half? You would have to dig out the soil under it, but the steeper the hill, the less you would have to dig.
I realize this is a tougher way of doing it, especially if you only have pick and shovel, but completely free in cost. Positioning would not be bad either as I would just have the thing delivered where you need it...unlevel...then jack the thing level. I did this last year on a
land clearing job. The owner wanted to position a container as I mentioned above while I had the equipment there. We had an excavator and bulldozer so we had to dig the hole first, but it worked. And I have moved several buildings, and jacking them is very easy to do. A container...just as easy.
A dry-laid rock wall is pretty fast and easy to build though. Don't be intimidated by that.