Expose DownloadAndExtractDockerImage as a util

Create a util sub cmd to add all utils that are handy for development
and already present in the luet codebase. We expose in this case `luet
util unpack` to unpack a docker image without a docker daemon running.

pkg/helpers/docker.go

@@ -0,0 +1,44 @@
+// Copyright © 2021 Ettore Di Giacinto <mudler@mocaccino.org>
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, see <http://www.gnu.org/licenses/>.
+
+package helpers
+
+import (
+	"os"
+
+	"github.com/mudler/luet/pkg/helpers/imgworker"
+	"github.com/pkg/errors"
+)
+
+// DownloadAndExtractDockerImage is a re-adaption
+// from genuinetools/img https://github.com/genuinetools/img/blob/54d0ca981c1260546d43961a538550eef55c87cf/pull.go
+func DownloadAndExtractDockerImage(temp, image, dest string) (*imgworker.ListedImage, error) {
+	defer os.RemoveAll(temp)
+	c, err := imgworker.New(temp)
+	if err != nil {
+		return nil, errors.Wrapf(err, "failed creating client")
+	}
+	defer c.Close()
+
+	listedImage, err := c.Pull(image)
+	if err != nil {
+		return nil, errors.Wrapf(err, "failed listing images")
+
+	}
+
+	os.RemoveAll(dest)
+	err = c.Unpack(image, dest)
+	return listedImage, err
+}