Copy file from Docker image to standard output

copy.go

package main

import (
	"archive/tar"
	"fmt"
	"io"
	"io/ioutil"
	"os"

	"github.com/docker/docker/api/types"
	"github.com/docker/docker/api/types/container"
	"github.com/docker/docker/client"
	"golang.org/x/net/context"
)

func main() {
	if len(os.Args) != 3 {
		fmt.Fprintf(os.Stderr, "Usage: %s IMAGE FILE\n", os.Args[0])
		os.Exit(1)
	}
	imageName := os.Args[1]
	filePath := os.Args[2]

	ctx := context.Background()
	cli, err := client.NewEnvClient()
	if err != nil {
		panic(err)
	}

	out, err := cli.ImagePull(ctx, imageName, types.ImagePullOptions{})
	if err != nil {
		panic(err)
	}
	defer out.Close()
	if _, err := ioutil.ReadAll(out); err != nil {
		panic(err)
	}

	info, err := cli.ContainerCreate(ctx, &container.Config{
		Image: imageName,
	}, nil, nil, "")
	if err != nil {
		panic(err)
	}

	tarStream, _, err := cli.CopyFromContainer(ctx, info.ID, filePath)
	if err != nil {
		panic(err)
	}
	tr := tar.NewReader(tarStream)
	if _, err := tr.Next(); err != nil {
		panic(err)
	}

	io.Copy(os.Stdout, tr)

	if err := cli.ContainerRemove(ctx, info.ID, types.ContainerRemoveOptions{}); err != nil {
		panic(err)
	}
}