Merge pull request #24339 from aaronlehmann/image-spec-fix

image spec: Correct set of acceptable characters in tags

image/spec/v1.1.md

@@ -87,7 +87,8 @@ This specification uses the following terms:
     <dd>
         A tag serves to map a descriptive, user-given name to any single image
         ID. Tag values are limited to the set of characters
-        <code>[a-zA-Z_0-9]</code>.
+        <code>[a-zA-Z0-9_.-]</code>, except they may not start with a <code>.</code>
+        or <code>-</code> character. Tags are limited to 127 characters.
     </dd>
     <dt>
         Repository