Implement compilation with images

A compilespec with an image defined (and no seed) can now be compiled,
and an artifact delta is provided at the end of the process.

pkg/helpers/archive.go

@@ -0,0 +1,62 @@
+// Copyright © 2019 Ettore Di Giacinto <mudler@gentoo.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 (
+	"io"
+	"os"
+
+	"github.com/docker/docker/pkg/archive"
+)
+
+func Tar(src, dest string) error {
+	out, err := os.Create(dest)
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+
+	fs, err := archive.Tar(src, archive.Uncompressed)
+	if err != nil {
+		return err
+	}
+	defer fs.Close()
+
+	_, err = io.Copy(out, fs)
+	if err != nil {
+		return err
+	}
+
+	err = out.Sync()
+	if err != nil {
+		return err
+	}
+	return err
+}
+
+// Untar just a wrapper around the docker functions
+func Untar(src, dest string, sameOwner bool) error {
+	in, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+
+	return archive.Untar(in, dest, &archive.TarOptions{
+		NoLchown:        !sameOwner,
+		ExcludePatterns: []string{"dev/"}, // prevent 'operation not permitted'
+	})
+}