title: create map[string]struct{} and assign a value
body: <p>I'm using the github.com/samalba/dockerclient and want to create a Container.
  So, the method is CreateContainer, which needs a ContainerConfig.</p>
  <p>The ContainerConfig is a struct. And there's a field Volumes, the type of which is type map[string] struct{}.</p>
  <p>I know that I could create such a map with <code>make(map[string]struct{})</code></p>
  <p>But how do I assign values to the map?</p>