Our dockerfiles no longer reference layers from other qemu images so
we can now use 'docker build' on them.

Also reinstate the caching that was disabled due to bad interactions
with certain runners. See commit 6ddc3dc7 ("tests/docker: don't use
BUILDKIT in GitLab either"). We now believe those issues to be fixed.

The COMMON_TAG needed to be fixed for the caching to work. The
docker.py script was not using the variable, but constructing the
correct URL directly.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230227151110.31455-2-farosas@suse.de>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230228190653.1602033-16-alex.bennee@linaro.org>