Add Docker output format

This will add a Dockerfile which will build the contents into an
image and then call `tinit` to start it.

This is fairly experimental, but is a prototype for other non
LinuxKit outputs. The container will need to run as `privileged`
as `runc` needs quite a few capabilities and `containerd` needs to
mount.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

cmd/moby/linuxkit.go

@@ -60,7 +60,7 @@ func ensureLinuxkitImage(name string) error {
 	}
 	// TODO pass through --pull to here
 	buf := new(bytes.Buffer)
-	buildInternal(m, buf, false)
+	buildInternal(m, buf, false, nil)
 	image := buf.Bytes()
 	kernel, initrd, cmdline, err := tarToInitrd(image)
 	if err != nil {