ADD enhancement: allow adding from another image

Rather than adding an additional buildfile instruction, this enhancement
will use a mock-URI prefix of "image://", followed by a image name, and
a ":<path>" suffix. (i.e. image://[registry[:port]]<name>[:tag]:<path> )

Such that if you have a container that has a built artifact, you can
copy it over to a new/different image.

```
FROM vbatts/minimal-runtime

ADD image://vbatts/fat-buildtime:/build/server /server

CMD "/server"
```

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)