doc: Expand what we say about building the docker container a bit

doc: Expand what we say about building the docker container a bit

First, try and be slightly clearer about what "buildx" is with respect
to the docker build process.

Second, now that we build the container for both amd64 and arm64, we
should document how to make a docker "builder" that has multiple nodes.
With this one node should be amd64 and one node arm64, and with
reasonably fast arm64 hardware this will be much quicker than using
QEMU.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>