docs/user_guides: Add support for rootless container support

Support for docker engine rootless containers feature has been added.
Documentation is updated to explain usage of the feature. This patch
updates documentation to reflect changes with new feature.

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: Ia1560b969730a153c7f0e7850eec8ad0040d7828