Use a build flag to disable resumable digests.

Signed-off-by: Andy Goldstein <agoldste@redhat.com>

docs/storage/layerwriter_resumable.go

@@ -0,0 +1,9 @@
+// +build !noresumabledigest
+
+package storage
+
+import "github.com/docker/distribution/digest"
+
+func (lw *layerWriter) setupResumableDigester() {
+	lw.resumableDigester = digest.NewCanonicalResumableDigester()
+}