Unify the get_layers calls across all implements of manifest schemas to ensure we have a common type returned

Also renames some methods to make it more clear what kind of information they return

image/docker/schema1.py

@@ -20,6 +20,7 @@ from jwt.utils import base64url_encode, base64url_decode
 
 from digest import digest_tools
 from image.docker import ManifestException
+from image.docker.types import ManifestImageLayer
 from image.docker.interfaces import ManifestInterface
 from image.docker.v1 import DockerV1Metadata
 
@@ -73,7 +74,7 @@ class InvalidSchema1Signature(ManifestException):
 
 
 class Schema1Layer(namedtuple('Schema1Layer', ['digest', 'v1_metadata', 'raw_v1_metadata',
-                                               'compressed_size', 'is_remote'])):
+                                               'compressed_size', 'is_remote', 'urls'])):
   """
   Represents all of the data about an individual layer in a given Manifest.
   This is the union of the fsLayers (digest) and the history entries (v1_compatibility).
@@ -298,6 +299,25 @@ class DockerSchema1Manifest(ManifestInterface):
       self._layers = list(self._generate_layers())
     return self._layers
 
+  def get_layers(self, content_retriever):
+    """ Returns the layers of this manifest, from base to leaf or None if this kind of manifest
+        does not support layers. """
+    for layer in self.layers:
+      created_datetime = None
+      try:
+        created_datetime = dateutil.parser.parse(layer.v1_metadata.created).replace(tzinfo=None)
+      except:
+        pass
+
+      yield ManifestImageLayer(layer_id=layer.v1_metadata.image_id,
+                               compressed_size=layer.compressed_size,
+                               is_remote=False,
+                               urls=None,
+                               command=layer.v1_metadata.command,
+                               blob_digest=layer.digest,
+                               created_datetime=created_datetime,
+                               internal_layer=layer)
+
   @property
   def blob_digests(self):
     return [str(layer.digest) for layer in self.layers]
@@ -356,7 +376,7 @@ class DockerSchema1Manifest(ManifestInterface):
                                     command, labels)
 
       compressed_size = v1_metadata.get('Size')
-      yield Schema1Layer(image_digest, extracted, metadata_string, compressed_size, False)
+      yield Schema1Layer(image_digest, extracted, metadata_string, compressed_size, False, None)
 
   @property
   def _payload(self):