Use pivot_root instead of chroot for chrootarchive

This fixes one issue with Docker running under a grsec kernel, which
denies chmod and mknod under chroot.

Note, if pivot_root fails it will still fallback to chroot.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

chrootarchive/chroot_unix.go

@@ -0,0 +1,12 @@
+// +build !windows,!linux
+
+package chrootarchive
+
+import "syscall"
+
+func chroot(path string) error {
+	if err := syscall.Chroot(path); err != nil {
+		return err
+	}
+	return syscall.Chdir("/")
+}