schema: Limit schemaVersion to 2 and enumerate mediaType

For manifests and manifest lists, to match the requirements from their
Markdown definitions.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

schema/image-manifest-schema.json

@@ -7,11 +7,16 @@
     "schemaVersion": {
       "description": "This field specifies the image manifest schema version as an integer",
       "id": "https://opencontainers.org/schema/image/manifest/schemaVersion",
-      "type": "integer"
+      "type": "integer",
+      "minimum": 2,
+      "maximum": 2
     },
     "mediaType": {
       "id": "https://opencontainers.org/schema/image/manifest/mediaType",
-      "$ref": "defs-image.json#/definitions/mediaType"
+      "type": "string",
+      "enum": [
+        "application/vnd.oci.image.manifest.v1+json"
+      ]
     },
     "config": {
       "$ref": "content-descriptor.json"

schema/manifest-list-schema.json

@@ -7,11 +7,16 @@
     "schemaVersion": {
       "description": "This field specifies the image manifest-list schema version as an integer",
       "id": "https://opencontainers.org/schema/image/manifest-list/schemaVersion",
-      "type": "integer"
+      "type": "integer",
+      "minimum": 2,
+      "maximum": 2
     },
     "mediaType": {
       "id": "https://opencontainers.org/schema/image/manifest-list/mediaType",
-      "$ref": "defs-image.json#/definitions/mediaType"
+      "type": "string",
+      "enum": [
+        "application/vnd.oci.image.manifest.list.v1+json"
+      ]
     },
     "manifests": {
       "type": "array",