Supposedly they auto-stop when inactive.
But I've tried multiple times and they don't stop, and it's not just Docker that prevents them from stopping.
I created a new sprite and installed ffmpeg. Then exited. Next day I run `sprite ls` and it's been running continuously for 23 hours.
No way to tell if I'm being billed for it or not.
And the per-hour pricing is extremely expensive.
So for now it's `sprite destroy -s spritename`.
Maybe I'll check it out again in a few months after the fly team has iterated on this a few times.