docker: Don't start a container that doesn't exist

Image building targets are dependencies of test running targets, so when
a docker image doesn't exist, it means it's skipped (due to dependency
checks in pre script). Therefore, skip the test too.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1468934445-32183-10-git-send-email-famz@redhat.com
