Looks like the old japaric images are bunk. We update our docker image
to be based on the new rustembedded images and configure cross to use
it.
Turns out that this wasn't due to a stale docker image, but rather, a
bug in cross: https://github.com/rust-embedded/cross/issues/357
We work around that bug by installing the master branch of cross. Sigh.
The docker image that the Linux binary is now built in does not have
ASCII doc installed, so setup Cross to point to my own image with those
tools installed.