Add netns option to --net switch

Allows attaching an existing network namespace to a container on create.
e.g. --net="netns:my-netns".

Closes #7455

Signed-off-by: Hugo Duncan <hugo@hugoduncan.org>
Add minimal description in networking.md

Signed-off-by: Hugo Duncan <hugo@hugoduncan.org>
Fix --net=netns:name test

Signed-off-by: Hugo Duncan <hugo@hugoduncan.org>
Pass full path to --net=netns:<path>

Signed-off-by: Hugo Duncan <hugo@hugoduncan.org>
Add default network example using --net=netns

Add an example for replicating the default docker network setup using
--net=netns:<path> to networking.md.

Signed-off-by: Hugo Duncan <hugo@hugoduncan.org>
Change netns:NAME in networking.md to netns:PATH

Signed-off-by: Hugo Duncan <hugo@hugoduncan.org>