README: Describe how to fetch pre-built images

Document usage of `docker run` which will automatically pull the image
and run it, now that the Dockerfile for `docker build` has been moved
to another repository,
    https://git.apertis.org/cgit/apertis-docker-images.git/

Apertis: https://phabricator.apertis.org/T4472


Signed-off-by: Andrew Lee (李健秋) <andrew.lee@collabora.co.uk>

Differential Revision: https://phabricator.apertis.org/D7434