def volumes_for_bind
      result = []
      each_volume do |container_path, local_path, permissions|
        result << "#{local_path}:#{container_path}:#{permissions}"
      end
      result
    end
Private: Generate the `Binds` argument for creating a container.

Given a hash of container_path => local_path in @volumes, generate an
array of "local_path:container_path:rw".