manifest: add annotations

This introduces OPTIONAL annotations, which are consistent with the
annotations in the runtime-spec.

While the spec does not exclude use of arbitrary fields, this OPTIONAL
property gives a guided place for manifest authors to isolate their
arbitrary metadata.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

manifest.md

@@ -89,6 +89,12 @@ A client will distinguish a manifest list from an image manifest based on the Co
 
         This OPTIONAL property specifies an array of strings, each specifying a mandatory CPU feature (for example `sse4` or `aes`).
 
+- **`annotations`** *hashmap*
+
+    This OPTIONAL property contains arbitrary metadata for the manifest list.
+    This information may be structured or unstructured.
+    Annotations are key-value maps.
+
 ## Example Manifest List
 
 *Example showing a simple manifest list pointing to image manifests for two platforms:*
@@ -118,7 +124,11 @@ A client will distinguish a manifest list from an image manifest based on the Co
         ]
       }
     }
-  ]
+  ],
+  "annotations": {
+    "key1": "value1",
+    "key2": "value2",
+  }
 }
 ```
 
@@ -182,6 +192,13 @@ The image manifest provides a configuration and a set of layers for a container
 
         The digest of the content, as defined by the [Registry V2 HTTP API Specificiation](https://docs.docker.com/registry/spec/api/#digest-parameter).
 
+- **`annotations`** *hashmap*
+
+    This OPTIONAL property contains arbitrary metadata for the manifest.
+    This information may be structured or unstructured.
+    Annotations are key-value maps.
+
+
 ## Example Image Manifest
 
 *Example showing an image manifest:*
@@ -210,7 +227,11 @@ The image manifest provides a configuration and a set of layers for a container
       "size": 73109,
       "digest": "sha256:ec4b8955958665577945c89419d1af06b5f7636b4ac3da7f12184802ad867736"
     }
-  ]
+  ],
+  "annotations": {
+    "key1": "value1",
+    "key2": "value2",
+  }
 }
 ```
 

schema/defs-image.json

@@ -94,6 +94,17 @@
           }
         }
       }
+    },
+    "annotations": {
+      "id": "https://opencontainers.org/schema/image/annotations",
+      "oneOf": [
+        {
+          "$ref": "defs.json#/definitions/mapStringString"
+        },
+        {
+          "type": "null"
+        }
+      ]
     }
   }
 }

schema/image-manifest-schema.json

@@ -21,6 +21,10 @@
       "items": {
         "$ref": "defs-image.json#/definitions/descriptor"
       }
+    },
+    "annotations": {
+      "id": "https://opencontainers.org/schema/image/manifest-list/annotations",
+      "$ref": "defs-image.json#/definitions/annotations"
     }
   },
   "required": [

schema/manifest-list-schema.json

@@ -18,6 +18,10 @@
       "items": {
         "$ref": "defs-image.json#/definitions/manifestDescriptor"
       }
+    },
+    "annotations": {
+      "id": "https://opencontainers.org/schema/image/manifest-list/annotations",
+      "$ref": "defs-image.json#/definitions/annotations"
     }
   },
   "required": [