Add CreateTar to image API

Add api call which uses go-container registry to create OCI images from
standard tar archives.

Consume new API when generating final images instead of docker building them
and adapts/add tests as necessary.

This change now allows to carry over xattrs to final images.

Fixes #266

pkg/api/core/image/create.go

@@ -0,0 +1,81 @@
+// 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 image
+
+import (
+	"io"
+	"os"
+
+	"github.com/google/go-containerregistry/pkg/name"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/empty"
+	"github.com/google/go-containerregistry/pkg/v1/mutate"
+	"github.com/google/go-containerregistry/pkg/v1/tarball"
+	"github.com/pkg/errors"
+)
+
+func imageFromTar(imagename string, r io.Reader) (name.Reference, v1.Image, error) {
+	newRef, err := name.ParseReference(imagename)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	layer, err := tarball.LayerFromReader(r)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	img, err := mutate.Append(empty.Image, mutate.Addendum{
+		Layer: layer,
+		History: v1.History{
+			CreatedBy: "luet",
+			Comment:   "Custom image",
+		},
+	})
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return newRef, img, nil
+}
+
+// CreateTar a imagetarball from a standard tarball
+func CreateTar(srctar, dstimageTar, imagename string) error {
+	f, err := os.Open(srctar)
+	if err != nil {
+		return errors.Wrap(err, "Cannot open "+srctar)
+	}
+	defer f.Close()
+
+	return CreateTarReader(f, dstimageTar, imagename)
+}
+
+// CreateTarReader a imagetarball from a standard tarball
+func CreateTarReader(r io.Reader, dstimageTar, imagename string) error {
+	dstFile, err := os.Create(dstimageTar)
+	if err != nil {
+		return errors.Wrap(err, "Cannot create "+dstimageTar)
+	}
+	defer dstFile.Close()
+
+	newRef, img, err := imageFromTar(imagename, r)
+	if err != nil {
+		return err
+	}
+
+	// NOTE: We might also stream that back to the daemon with daemon.Write(tag, img)
+	return tarball.Write(newRef, img, dstFile)
+}