api/image/list: Return Containers count

This parameter was already supported for some time in the backend (for
purposes related to docker system prune). It was also already present in
the imagetypes.ListOptions but was never actually handled by the client.

Make it available by default in the response.

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

api/server/router/image/image_routes.go

@@ -459,6 +459,7 @@ func (ir *imageRouter) getImagesJSON(ctx context.Context, w http.ResponseWriter,
 	useNone := versions.LessThan(version, "1.43")
 	withVirtualSize := versions.LessThan(version, "1.44")
 	noDescriptor := versions.LessThan(version, "1.48")
+	noContainers := versions.LessThan(version, "1.51")
 	for _, img := range images {
 		if useNone {
 			if len(img.RepoTags) == 0 && len(img.RepoDigests) == 0 {
@@ -479,6 +480,9 @@ func (ir *imageRouter) getImagesJSON(ctx context.Context, w http.ResponseWriter,
 		if noDescriptor {
 			img.Descriptor = nil
 		}
+		if noContainers {
+			img.Containers = -1
+		}
 	}
 
 	return httputils.WriteJSON(w, http.StatusOK, images)