Add standard base image annotations

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

annotations.md

@@ -40,6 +40,19 @@ 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)
+  * For the purposes of this annotation, "base image" SHOULD be defined as an image of N layers, that correspond directly to layers 0-N of the image with this annotation, to form the initial root filesystem of the image.
+* **org.opencontainers.image.base.ref.name** Name of the reference for the base image (string)
+  * For the purposes of this annotation, "base image" SHOULD be defined as an image of N layers, that correspond directly to layers 0-N of the image with this annotation, to form the initial root filesystem of the image.
+  * SHOULD be a tag reference
+  * Character set of the value SHOULD conform to alphanum of `A-Za-z0-9` and separator set of `-._:@/+`
+  * The reference must match the following [grammar](considerations.md#ebnf):
+    ```
+    ref       ::= component ("/" component)*
+    component ::= alphanum (separator alphanum)*
+    alphanum  ::= [A-Za-z0-9]+
+    separator ::= [-._:@+] | "--"
+    ```
 
 ## Back-compatibility with Label Schema