manifest: support urls for layers

Signed-off-by: Antonio Murdaca <[email protected]>

Author: runcom

manifest.md

@@ -191,6 +191,10 @@ Unlike the [Manifest List](#manifest-list), which contains information about a s
 
         This REQUIRED property is the digest of the content, as defined by the [Descriptor](descriptor.md) digest format.
 
+    - **`urls`** *array*
+
+       For an ordinary layer, this is empty, and the layer contents can be retrieved directly from the registry. For a layer with mediatype of application/vnd.docker.image.rootfs.foreign.diff.tar.gzip, this contains a non-empty list of URLs from which this object can be downloaded.
+
 - **`annotations`** *hashmap*
 
     This OPTIONAL property contains arbitrary metadata for the manifest list.