我如何获得docker build登录下载进度标准输出

Question

FROM centos:6.7

RUN ls -al

WORKDIR /

CMD "echo hi"

this is simple Dockerfile

I will create an image with this dockerfile and i'm not use shell cmd

i'm using golang "exec" package and here is code

func execCommand(buildCmd string) {

cmdArgs := strings.Fields(buildCmd)

cmd := exec.Command(cmdArgs[0], cmdArgs[1:len(cmdArgs)]...)
cmdReader, _ := cmd.StdoutPipe()
cmdReaderErr, _ := cmd.StderrPipe()

scanner := bufio.NewScanner(cmdReader)
scannerErr := bufio.NewScanner(cmdReaderErr)

cmd.Start()
go func() {

    for scanner.Scan() {
        fmt.Println("docker build out | ", scanner.Text())
    }

    if scannerErr.Scan() {
        fmt.Print("docker build out err | ", scannerErr.Text())
    }
    fmt.Println("end end end end ")

    }()
}

this code is work well but here's the problem

1. docker build out | Sending build context to Docker daemon 30.36MB

2. docker build out | Step 1/4 : FROM centos:6.7

3. docker build out | 6.7: Pulling from library/centos

4. docker build out | cbddbc0189a0: Pulling fs layer

4 line Pulling fs layer has can't change download progress bar

and suddenly showing next line

1. docker build out | Sending build context to Docker daemon 30.36MB

2. docker build out | Step 1/4 : FROM centos:6.7

3. docker build out | 6.7: Pulling from library/centos

4. docker build out | cbddbc0189a0: Pulling fs layer

5. docker build out | cbddbc0189a0: Verifying Checksum

6. docker build out | cbddbc0189a0: Download complete

7. docker build out | cbddbc0189a0: Pull complete

8. docker build out | Digest: sha256:4c952fc7d30ed134109c769387313ab864711d1bd8b4660017f9d27243622df1

9. docker build out | Status: Downloaded newer image for centos:6.7

this is shell command docker build log

docker build -t test:v1 .

Sending build context to Docker daemon 30.36MB

Step 1/4 : FROM centos:6.7

6.7: Pulling from library/centos

cbddbc0189a0: Downloading [===============> ] 20.92MB/67.81MB

golang stdout dosen't show Downloading [===============> ] 20.92MB/67.81MB

how can i show this stdout?