Add 'ancestor' ps filter for image

Makes it possible to filter containers by image, using
--filter=ancestor=busybox and get all the container running busybox
image and image based on busybox (to the bottom).

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

docs/reference/commandline/ps.md

@@ -50,6 +50,7 @@ The currently supported filters are:
 * name (container's name)
 * exited (int - the code of exited containers. Only useful with `--all`)
 * status (created|restarting|running|paused|exited)
+* ancestor (`<image-name>[:<tag>]`,  `<image id>` or `<image@digest>`) - filters containers that were created from the given image or a descendant.
 
 
 #### Label