Pour it in the compost. That much in a decent sized compost pile shouldn't be a problem, as long as the pile is not too wet. If you have varmint problems, it may be an attractant for a short time. Cover the pile with fresh material (leaves, etc.) which will keep odors in and critters out.

"They" say to avoid dairy in the compost, but it's not because it's not compostable, only because it can attract pests. Proceed with common sense and caution and you'll be fine.

The fats in milk are difficult for the bacteria in a compost pile to digest, which is why "they" recommend against putting dairy products in the compost bin.
I know someone that poured some sour milk into his compost pile and still could smell sour milk when he put that compost on his garden 6 months later.

On the other hand I've put bad milk on the garden and in the compost and it's soaked in instantly and not smelled at all. 1/2 US gallon is not going to make any difference. And if were to smell, isn't that preferable outdoors than down your sink? Your call.

sour milk actually speed composting process (from reading and my experience)