code: func dispatchArg(d dispatchRequest, c *instructions.ArgCommand) error {
  
  	commitStr := "ARG " + c.Key
  	if c.Value != nil {
  		commitStr += "=" + *c.Value
  	}
  
  	d.state.buildArgs.AddArg(c.Key, c.Value)
  	return d.builder.commit(d.state, commitStr)
  }
docs: // ARG name[=value]
  //
  // Adds the variable foo to the trusted list of variables that can be passed
  // to builder using the --build-arg flag for expansion/substitution or passing to 'run'.
  // Dockerfile author may optionally set a default value of this variable.