docker: Build base container for Crunch jobs.
crunch-job: Support runtime Docker image.

This adds support for the allow_other mount option in arv-mount to
make the Keep mount available to the container.
crunch: Install Docker images on demand.

Ward requested this to simplify the installation and testing
procedure.  This is sort of temporary scaffolding; we expect that in
the future Docker images will be stored in, and read from, Keep.
Merge branch '2492-docker-crunch-jobs'

Closes #2492.