I tried this, and it seems it is already up to date.
# apt install docker-ce docker-ce-cli
Reading package lists... Done
Building dependency tree
Reading state information... Done
docker-ce is already the newest version (5:20.10.7~3-0~ubuntu-xenial).
docker-ce-cli is already the newest version (5:20.10.7~3-0~ubuntu-xenial).
docker-ce-cli set to manually installed.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Switched docker over to overlay2 (as described Change the Docker storage backend), deleted the old aufs directory, but rebuild still fails the same way.
Then did do-release-upgrade which will let me do this now since I am no longer using aufs.
And after all that, it still tells me:
WARNING: Docker version 20.10.7 deprecated, recommend upgrade to 24.0.7 or newer.
and still fails in the same way.
Then another do-release-upgrade -d to update to 24 (which sounds concerning The rocky road to upgrading Ubuntu Linux 24.04 | ZDNET). But hey, I’ve got backups and a snapshot and the server is already down…
Ok, but now docker is still version 20.
Forcibly update docker to version 24 (Ubuntu | Docker Docs).
Rebuild again.
And it rebuilt. Wow.
Well, that was fun. Or something.
Here is hoping all that upgrading hasn’t broken anything else, but it looks to be back working and updated - cross fingers.