pulp_publish: only remove each docker_image unit once
pulp_publish: only remove each docker_image unit once

Signed-off-by: Tim Waugh <fe049355006bf7fdd11c5e47ffe7a5032af30704@redhat.com>

projectatomic/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor,fr34k8/atomic-reactor,jarodwilson/atomic-reactor,maxamillion/atomic-reactor,projectatomic/atomic-reactor,vrutkovs/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,jarodwilson/atomic-reactor