code: func dispatchEnv(d dispatchRequest, c *instructions.EnvCommand) error {
  	runConfig := d.state.runConfig
  	commitMessage := bytes.NewBufferString("ENV")
  	for _, e := range c.Env {
  		name := e.Key
  		newVar := e.String()
  
  		commitMessage.WriteString(" " + newVar)
  		gotOne := false
  		for i, envVar := range runConfig.Env {
  			envParts := strings.SplitN(envVar, "=", 2)
  			compareFrom := envParts[0]
  			if shell.EqualEnvKeys(compareFrom, name) {
  				runConfig.Env[i] = newVar
  				gotOne = true
  				break
  			}
  		}
  		if !gotOne {
  			runConfig.Env = append(runConfig.Env, newVar)
  		}
  	}
  	return d.builder.commit(d.state, commitMessage.String())
  }
docs: // ENV foo bar
  //
  // Sets the environment variable foo to bar, also makes interpolation
  // in the dockerfile available from the next statement on via ${foo}.
  //