fix dockerhub readme

[juniperlsimonis]

not sure why it is showing what it is -- an old top-level readme, lagging behind main repo
we have a separate readme in the docker folder and the updated readme at the toplevel, yet we're getting an old (buggy) version

[ethanwhite]

Pretty sure the dockerhub README is a copy paste situation

[juniperlsimonis]

by default it is, but there are well established ways to have it autoupdate and i used to have one in place but it broke with some change a while back i think

(e.g., https://github.com/marketplace/actions/update-container-description-action)

[juniperlsimonis]

there's some discussion about when this will get pulled into an official workflow on this issue -- docker/build-push-action#21

seems like they are actively trying to make it a core functionality

[ethanwhite]

Way ahead of me as usual 😄