Apparently some newer docker versions crash if there is no "repository"
file in the resulting image. Its syntax seems simple enough, described
in https://github.com/moby/moby/blob/master/image/spec/v1.md
Reported-by: Matthew Singletary <matt.singletary@gmail.com>
This got... long... and complicated. But I think it's a really good
demonstration of getting redo to do complicated things elegantly. At
least, I hope it is.