Add a new "image" mount type to `--mount`. The source of the mount is
the name or ID of an image. The destination is the path inside the
container. Image mounts further support an optional `rw,readwrite`
parameter which if set to "true" will yield the mount writable inside
the container. Note that no changes are propagated to the image mount
on the host (which in any case is read only).
Mounts are overlay mounts. To support read-only overlay mounts, vendor
a non-release version of Buildah.
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>
