c8d/prune: Default `dangling` filter to true

If no `dangling` filter is specified, prune should only delete dangling
images.

This wasn't visible by doing `docker image prune` because the CLI
explicitly sets this filter to true.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

daemon/containerd/image_prune.go

@@ -42,7 +42,7 @@ func (i *ImageService) ImagesPrune(ctx context.Context, fltrs filters.Args) (*ty
 		return nil, err
 	}
 
-	danglingOnly, err := fltrs.GetBoolOrDefault("dangling", false)
+	danglingOnly, err := fltrs.GetBoolOrDefault("dangling", true)
 	if err != nil {
 		return nil, err
 	}