code: func Auth(username, password string) registry.Option {
  	return func(o *registry.Options) {
  		if o.Context == nil {
  			o.Context = context.Background()
  		}
  		o.Context = context.WithValue(o.Context, authKey{}, &authCreds{Username: username, Password: password})
  	}
  }
docs: // Auth allows you to specify username/password