Outputs to stdout the manifest for an image from dockerhub registry

docker-get-manifest.go

package main

import (
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"net/url"
	"os"
	"strings"
)

func printManifest(image string) error {
	parts := strings.SplitN(image, "@", 2)
	if len(parts) < 2 {
		return fmt.Errorf("missing tag in image name %s", image)
	}
	name, tag := parts[0], parts[1]

	token, err := getToken("registry.docker.io", "repository:"+name+":pull")
	if err != nil {
		return err
	}

	req, err := http.NewRequest("GET", fmt.Sprintf("https://index.docker.io/v2/%s/manifests/%s", name, tag), nil)
	if err != nil {
		return err
	}
	req.Header.Add("Authorization", "Bearer "+token)
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	io.Copy(os.Stdout, resp.Body)
	return nil
}

func getToken(service, scope string) (string, error) {
	// We need to get a token
	v := url.Values{}
	v.Set("service", service)
	v.Set("scope", scope)
	resp, err := http.Get("https://auth.docker.io/token?" + v.Encode())
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()
	t := struct {
		Token string `json:"token"`
	}{}
	err = json.NewDecoder(resp.Body).Decode(&t)
	return t.Token, err
}

func main() {
	for _, image := range os.Args[1:] {
		err := printManifest(image)
		if err != nil {
			log.Fatal(err)
		}
	}
}