Adds functionality to parse and return network attachment spec
information. Network attachment tasks are phony tasks created in
swarmkit to deal with unmanaged containers attached to swarmkit. Before
this change, attempting `docker inspect` on the task id of a network
attachment task would result in an empty task object. After this change,
a full task object is returned
Fixes#26548 the correct way.
Signed-off-by: Drew Erny <drew.erny@docker.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
