Add support for pushing and pulling schema 2 manifests with remote layers

This is required for windows image support

test/registry/protocol_v2.py

@@ -209,13 +209,15 @@ class V2Protocol(RegistryProtocol):
     builder = DockerSchema2ManifestBuilder()
     for image in images:
       checksum = 'sha256:' + hashlib.sha256(image.bytes).hexdigest()
-      blobs[checksum] = image.bytes
+
+      if image.urls is None:
+        blobs[checksum] = image.bytes
 
       # If invalid blob references were requested, just make it up.
       if options.manifest_invalid_blob_references:
         checksum = 'sha256:' + hashlib.sha256('notarealthing').hexdigest()
 
-      builder.add_layer(checksum, len(image.bytes))
+      builder.add_layer(checksum, len(image.bytes), urls=image.urls)
 
     config = {
       "os": "linux",
@@ -245,6 +247,8 @@ class V2Protocol(RegistryProtocol):
     builder = DockerSchema1ManifestBuilder(namespace, repo_name, tag_name)
 
     for image in reversed(images):
+      assert image.urls is None
+
       checksum = 'sha256:' + hashlib.sha256(image.bytes).hexdigest()
       blobs[checksum] = image.bytes
 
@@ -498,12 +502,16 @@ class V2Protocol(RegistryProtocol):
 
       # Verify the layers.
       for index, layer in enumerate(manifest.layers):
+        # If the layer is remote, then we expect the blob to *not* exist in the system.
+        expected_status = 404 if images[index].urls else 200
         result = self.conduct(session, 'GET',
                               '/v2/%s/blobs/%s' % (self.repo_name(namespace, repo_name),
                                                    layer.digest),
-                              expected_status=200,
+                              expected_status=expected_status,
                               headers=headers)
-        assert result.content == images[index].bytes
+        
+        if expected_status == 200:
+          assert result.content == images[index].bytes
 
     return PullResult(manifests=manifests, image_ids=image_ids)