Try to determine the bind mount file for dockerinit

This patch adds code to the contrib/docker_cr.sh helper script for trying
to determine the external bind mount file for dockerinit if the user
has not explicitly specified it via the DOCKERINIT_BINARY environment
variable.

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>