Add standard base image annotations

Signed-off-by: Jason Hall <[email protected]>

annotations.md

@@ -40,6 +40,14 @@ This specification defines the following annotation keys, intended for but not l
     ```
 * **org.opencontainers.image.title** Human-readable title of the image (string)
 * **org.opencontainers.image.description** Human-readable description of the software packaged in the image (string)
+* **org.opencontainers.image.base.digest** [Digest](descriptor.md#digests) of the image this image is based on (string)
+  * This SHOULD be the immediate image sharing zero-indexed layers with the image, such as from a Dockerfile `FROM` statement.
+  * This SHOULD NOT reference any other images used to generate the contents of the image (e.g., multi-stage Dockerfile builds).
+* **org.opencontainers.image.base.ref.name** Image reference of the image this image is based on (string)
+  * This SHOULD be image references in the format defined by [distribution/distribution](https://github.com/distribution/distribution/blob/main/reference/reference.go).
+  * This SHOULD be a fully qualified reference name, without any assumed default registry. (e.g., `index.docker.io/some/image` instead of `some/image`).
+  * This SHOULD be the immediate image sharing zero-indexed layers with the image, such as from a Dockerfile `FROM` statement.
+  * This SHOULD NOT reference any other images used to generate the contents of the image (e.g., multi-stage Dockerfile builds).
 
 ## Back-compatibility with Label Schema