Link a build's bases into its repository before pushing it (issue 694)

A runtime on containerd's image store, the default on a fresh docker 29,
need not hold the compressed layers of a base it built FROM: a layer it
had unpacked already is never fetched. Its push then fails on that layer
although the registry holds it under the base's mirror. A push reads
locally only what the target repository lacks, so mount the base's blobs
there first.

internal/builder/reuse.go

@@ -257,6 +257,13 @@ func (s scratchPublisher) PublishArchive(ctx context.Context, repository string,
 	return s.to.PublishArchive(ctx, ScratchPrefix+repository, body, digest)
 }
 
+func (s scratchPublisher) LendBases(ctx context.Context, repository string, bases []string) (int, error) {
+	if l, can := s.to.(BaseLender); can {
+		return l.LendBases(ctx, ScratchPrefix+repository, bases)
+	}
+	return 0, nil
+}
+
 func (s scratchPublisher) MirrorImage(ctx context.Context, from, repository string) (string, error) {
 	m, can := s.to.(Mirrorer)
 	if !can {