fix(server): Use uncompressed tarball hashes in image config

Docker expects hashes of compressed tarballs in the manifest (as these
are used to fetch from the content-addressable layer store), but for
some reason it expects hashes in the configuration layer to be of
uncompressed tarballs.

To achieve this an additional SHA256 hash is calculcated while
creating the layer tarballs, but before passing them to the gzip
writer.

In the current constellation the symlink layer is first compressed and
then decompressed again to calculate its hash. This can be refactored
in a future change.

tools/nixery/server/builder/archive.go

@@ -10,6 +10,8 @@ package builder
 import (
 	"archive/tar"
 	"compress/gzip"
+	"crypto/sha256"
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -19,26 +21,31 @@ import (
 
 // Create a new compressed tarball from each of the paths in the list
 // and write it to the supplied writer.
-func packStorePaths(l *layers.Layer, w io.Writer) error {
+//
+// The uncompressed tarball is hashed because image manifests must
+// contain both the hashes of compressed and uncompressed layers.
+func packStorePaths(l *layers.Layer, w io.Writer) (string, error) {
+	shasum := sha256.New()
 	gz := gzip.NewWriter(w)
-	t := tar.NewWriter(gz)
+	multi := io.MultiWriter(shasum, gz)
+	t := tar.NewWriter(multi)
 
 	for _, path := range l.Contents {
 		err := filepath.Walk(path, tarStorePath(t))
 		if err != nil {
-			return err
+			return "", err
 		}
 	}
 
 	if err := t.Close(); err != nil {
-		return err
+		return "", err
 	}
 
 	if err := gz.Close(); err != nil {
-		return err
+		return "", err
 	}
 
-	return nil
+	return fmt.Sprintf("sha256:%x", shasum.Sum([]byte{})), nil
 }
 
 func tarStorePath(w *tar.Writer) filepath.WalkFunc {