Fix broken JSON support in cli/command/formatter

How to test (it should not print `{}`, and just returns JSON with the actual data):

   $ docker images --format '{{json .}}'
   $ docker container stats --format '{{json .}}'

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

command/formatter/disk_usage.go

@@ -158,6 +158,10 @@ type diskUsageImagesContext struct {
 	images    []*types.ImageSummary
 }
 
+func (c *diskUsageImagesContext) MarshalJSON() ([]byte, error) {
+	return marshalJSON(c)
+}
+
 func (c *diskUsageImagesContext) Type() string {
 	c.AddHeader(typeHeader)
 	return "Images"
@@ -209,6 +213,10 @@ type diskUsageContainersContext struct {
 	containers []*types.Container
 }
 
+func (c *diskUsageContainersContext) MarshalJSON() ([]byte, error) {
+	return marshalJSON(c)
+}
+
 func (c *diskUsageContainersContext) Type() string {
 	c.AddHeader(typeHeader)
 	return "Containers"
@@ -273,6 +281,10 @@ type diskUsageVolumesContext struct {
 	volumes []*types.Volume
 }
 
+func (c *diskUsageVolumesContext) MarshalJSON() ([]byte, error) {
+	return marshalJSON(c)
+}
+
 func (c *diskUsageVolumesContext) Type() string {
 	c.AddHeader(typeHeader)
 	return "Local Volumes"