CI: Allow a mirror to be specified for Docker Hub

To conserve bandwidth and potentially avoid rate limits, allow a local
mirror of Docker Hub to be specified globally. The default value is
unchanged.

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