docker.py: fix fetching of FROM layers

docker.py: fix fetching of FROM layers

This worked on a system that was already bootstrapped because the
stage 2 images already existed even if they wouldn't be used. What we
should have pulled down was the FROM line containers first because
building on gitlab doesn't have the advantage of using our build
system to build the pre-requisite bits.

We still pull the image we want to build just in case we can use the
cached data.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200713200415.26214-4-alex.bennee@linaro.org>