It's quite frustrating to sit down for the day, and something that worked the day before no longer works. I was hitting the following problem with
---> Running in 143123d54e5b hcsshim::CreateComputeSystem 143123d54e5b8c3b028bfad97141a2b1afc6ffa04889a4213f62b2d0daa9f91c: Logon failure: the user has not been granted the requested logon type at this computer.
A reboot seemed to solve it, but I think that was more luck than judgement. Thankfully I found a better answer in https://github.com/docker/for-win/issues/1056#issuecomment-401810678, namely, restarting the "Hyper-V Virtual Machine Management" service.