Flush stdin from within chroot archive

This makes sure that we don't buffer in memory and that we also flush
stdin from diff as well as untar.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Conflicts:
	pkg/chrootarchive/diff.go

chrootarchive/init.go

@@ -2,6 +2,8 @@ package chrootarchive
 
 import (
 	"fmt"
+	"io"
+	"io/ioutil"
 	"os"
 
 	"github.com/docker/docker/pkg/reexec"
@@ -16,3 +18,9 @@ func fatal(err error) {
 	fmt.Fprint(os.Stderr, err)
 	os.Exit(1)
 }
+
+// flush consumes all the bytes from the reader discarding
+// any errors
+func flush(r io.Reader) {
+	io.Copy(ioutil.Discard, r)
+}