Docker/CI: Update to "focal" and latest build

Move us up to being based on Ubuntu 20.04 "focal" and the latest tag
from Ubuntu for this release.  For this, we make sure that "python" is
now python3 but still include python2.7 for the rx51 qemu build as that
is very old and does not support python3.

Signed-off-by: Tom Rini <trini@konsulko.com>