code: func (c *ContainerServer) GetContainerRwSize(containerID string) (int64, error) {
  	container, err := c.store.Container(containerID)
  	if err != nil {
  		return 0, err
  	}
  
  	// Get the size of the top layer by calculating the size of the diff
  	// between the layer and its parent.  The top layer of a container is
  	// the only RW layer, all others are immutable
  	layer, err := c.store.Layer(container.LayerID)
  	if err != nil {
  		return 0, err
  	}
  	return c.store.DiffSize(layer.Parent, layer.ID)
  }
docs: // GetContainerRwSize Gets the size of the mutable top layer of the container