You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: code.go
    path of file: ./repos/nanobox/processors/code
    
    the code of the file until where you have to start completion: // Package code ...
package code

import (
	"time"

	"github.com/jcelliott/lumber"
	"github.com/nanobox-io/golang-docker-client"
	"github.com/nanobox-io/nanobox-boxfile"

	"github.com/nanobox-io/nanobox/util"
	"github.com/nanobox-io/nanobox/util/config"
	"github.com/nanobox-io/nanobox/util/display"
)

// these constants represent different potential names a service can have
const (
	BUILD = "build"
)

// t
    
func pullBuildImage() (string, error) {
	// extract the build image from the boxfile
	buildImage := buildImage()

	if docker.ImageExists(buildImage) {
		return buildImage, nil
	}

	display.StartTask("Pulling %s image", buildImage)
	defer display.StopTask()

	// generate a docker percent display
	dockerPercent := &display.DockerPercentDisplay{
		Output: display.NewStreamer("info"),
		// Prefix: buildImage,
	}

	// pull the build image
	imagePull := func() error {
		_, err := docker.ImagePull(buildImage, dockerPercent)
		return err
	}
	if err := util.Retry(imagePull, 5, time.Second); err != nil {
		lumber.Error("code:pullBuildImage:docker.ImagePull(%s, nil): %s", buildImage, err.Error())
		display.ErrorTask()
		return "", util.ErrorAppend(err, "failed to pull docker image (%s)", buildImage)
	}

	return buildImage, nil
}