The layers in the Dockerfile that depend on downloading large files from
external sources, doesn't have to run every day, or at every PR. We
tried using Docker's caching configuration, but it's not as reliable as
we'd have liked.
A separate base image lends us much more control over the how long we
cache the downloaded files and how often we redo this.
This PR only _adds_ the base image. It doesn't change anything in the
build of the existing Docker image. That'll happen once we have the base
images for `release` and `master` already present on DockerHub.
