Btrfs vs the Docker Borganism
1) Finicky docker+btrfs snapshot/dedupe management, or
2) Delete docker.
I chose 2).
Not that I could not do 1) (and in fact did a run-through to see how it went) or that I do not occasionally kind of appreciate the joy of tuning filesystem parameters. But it’s 2021 and I don’t want to have to do this on my working laptop. Personal filesystems should not require manual maintenance for basic functions, like preserving free space.