Make command optional in schema 2 manifests (as per OCI spec) and pull out additional information

Also updates the manifest view page to show the comment or digest if there is no command defined

endpoints/api/manifest.py

@@ -37,10 +37,12 @@ def _label_dict(label):
 def _layer_dict(manifest_layer, index):
   # NOTE: The `command` in the layer is either a JSON string of an array (schema 1) or
   # a single string (schema 2). The block below normalizes it to have the same format.
-  try:
-    command = json.loads(manifest_layer.command)
-  except (TypeError, ValueError):
-    command = [manifest_layer.command]
+  command = None
+  if manifest_layer.command:
+    try:
+      command = json.loads(manifest_layer.command)
+    except (TypeError, ValueError):
+      command = [manifest_layer.command]
 
   return {
     'index': index,
@@ -48,6 +50,8 @@ def _layer_dict(manifest_layer, index):
     'is_remote': manifest_layer.is_remote,
     'urls': manifest_layer.urls,
     'command': command,
+    'comment': manifest_layer.comment,
+    'author': manifest_layer.author,
     'blob_digest': str(manifest_layer.blob_digest),
     'created_datetime': format_date(manifest_layer.created_datetime),
   }