By not using "--format" with docker images command.
The option is not available on RHEL 7 docker command. Use an awk
matching command instead.
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1470202928-3392-1-git-send-email-famz@redhat.com>