script/build-with-container.py: build builder image with --pull=always

script/build-with-container.py: build builder image with --pull=always

Construct the builder image using the --pull=always flag to initiate a
pull of the base image (centos, ubuntu, etc) in order to avoid using a
stale base image. Since the script automatically (by default) avoids
building if a matching tag is in local container storage it is handy to
use a fresh base when it *is* time to build something. Otherwise, you
end up in a situation like I sometimes do - using a months old base
unintentionally.

Signed-off-by: John Mulligan <jmulligan@redhat.com>