Implement docker pull with standalone client lib.

Signed-off-by: David Calavera <david.calavera@gmail.com>

api/client/lib/image_pull.go

@@ -0,0 +1,34 @@
+package lib
+
+import (
+	"io"
+	"net/http"
+	"net/url"
+
+	"github.com/docker/docker/api/types"
+)
+
+// ImagePull request the docker host to pull an image from a remote registry.
+// It executes the privileged function if the operation is unauthorized
+// and it tries one more time.
+// It's up to the caller to handle the io.ReadCloser and close it properly.
+func (cli *Client) ImagePull(options types.ImagePullOptions, privilegeFunc RequestPrivilegeFunc) (io.ReadCloser, error) {
+	query := url.Values{}
+	query.Set("fromImage", options.ImageID)
+	if options.Tag != "" {
+		query.Set("tag", options.Tag)
+	}
+
+	resp, err := cli.tryImageCreate(query, options.RegistryAuth)
+	if resp.statusCode == http.StatusUnauthorized {
+		newAuthHeader, privilegeErr := privilegeFunc()
+		if privilegeErr != nil {
+			return nil, privilegeErr
+		}
+		resp, err = cli.tryImageCreate(query, newAuthHeader)
+	}
+	if err != nil {
+		return nil, err
+	}
+	return resp.body, nil
+}