Properly handle the empty layer when pushing schema 2 manifests

Docker doesn't send us the contents of this layer, so we are forced to synthesize it ourselves

image/docker/schemautil.py

@@ -1,4 +1,8 @@
 import json
+import tarfile
+
+from cachetools import lru_cache
+from io import BytesIO
 
 from image.docker.interfaces import ContentRetriever
 
@@ -22,3 +26,12 @@ class ContentRetrieverForTesting(ContentRetriever):
     digests = {}
     digests[digest] = padded_string
     return ContentRetrieverForTesting(digests)
+
+
+@lru_cache(maxsize=1)
+def generate_empty_layer_data():
+  """ Generates the layer data for an "empty" layer. """
+  with BytesIO() as f:
+    tar_file = tarfile.open(fileobj=f, mode='w|gw')
+    tar_file.close()
+    return f.getvalue()