ADD enhancement: docs for image://

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
ADD enhancement: nop Cmd for the <src> config

A command is required for the context setup as the <src> image.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)