Since the dockerfile now requires buildkit, let's just use buildx can
bootstrap itself into even an old version of Docker which does not
support buildkit.
This also decouples the Dockerfile/build from the version of Docker
which is installed.
One major downside:
If buildx needs to setup a container driver (ie, docker does not support
buildkit), the `make shell` target (and others which call
`DOCKER_RUN_DOCKER`) must export the image from buildkit and into
docker. This added an extra 70s to a full build for me (agan only for targets
which call `DOCKER_RUN_DOCKER`) and 40s on a rebuild (with no changes).
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
