Add docker runtime dependency on kernel-module-xt-nat. This module
enables docker port mapping.

To install docker in the target image, add the following
configuration in local.conf:

    DISTRO_FEATURES:append = " virtualization"

Issue-Id: SCM-4875
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I87cc728bcc8638c41514ca22beb2c094446b8348
Signed-off-by: Jiacheng Tang <jiacheng.tang@arm.com>