Damage from flea beetles.
If limited to those lower leaves, may not need to do anything.

Flea beetles

I had a feeling that might be what it was, but I've never actually seen one of the little guys, so I didn't want to assume it was a pest and have it actually be an infection of some kind. Three out of my four tomatoes have received pretty extensive damage on the bottom few layers of leaves, but the plants, overall, appear to be really healthy (a welcome change for me). It looks like there's not much one can do about them, anyway. Thank you very much for the responses. Here's hoping, after two years of pretty abysmal failure, I will have a successful little container garden!