Fix support for registry auth with Dockerfile build.

Docker-DCO-1.1-Signed-off-by: Jake Moshenko <jake@devtable.com> (github: jakedt)
Roll the API rev to version 1.9. Make the versions before 1.8 backward compatible with the old X-Registry-Auth header.

Docker-DCO-1.1-Signed-off-by: Jake Moshenko <jake@devtable.com> (github: jakedt)
Add a comment which explains why we're parsing both headers if provided.

Docker-DCO-1.1-Signed-off-by: Jake Moshenko <jake@devtable.com> (github: jakedt)