Add a new "is-task" ps filter

This makes it easier to list containers that are part of a task
(swarm mode) and those who are not.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

man/docker-ps.1.md

@@ -32,6 +32,7 @@ the running containers.
    - status=(created|restarting|running|paused|exited|dead)
    - name=<string> a container's name
    - id=<ID> a container's ID
+   - is-task=(true|false) - containers that are a task (part of a service managed by swarm)
    - before=(<container-name>|<container-id>)
    - since=(<container-name>|<container-id>)
    - ancestor=(<image-name>[:tag]|<image-id>|<image@digest>) - containers created from an image or a descendant.