cmd-push-container-manifest: change image key schema

The `oscontainer` and `base-oscontainer` keys should follow the same
schema. Currently, the former has a `digest` field, while the other one
does not. Tweak `cosa push-container-manifest` and `cosa push-container`
so that they follows the new schema. (Though note the latter command
will be deleted soon).

To keep previous 4.12 `meta.json` files valid, this loosens the `image`
schema definition so that `digest` is now optional. Once we branch for
4.12, we will undo this change so that it becomes required again.

Fixes #3122

(cherry picked from commit 738cb39)

jlebon: I skipped backporting the changes to the schema since we don't
        need to be as flexible for 4.11. I also skipped the similar
        change to `cmd-push-container` since we're not planning to use
        that at all in the new pipeline.
(cherry picked from commit b5a3842)
(cherry picked from commit 2af1bd0)

src/cmd-push-container-manifest

@@ -68,7 +68,10 @@ def main():
 
         # Update the meta.json in each build/arch metadata
         for _, buildmeta in buildmetas.items():
-            buildmeta[args.metajsonname] = {'image': f"{args.repo}@{digest}"}
+            buildmeta[args.metajsonname] = {
+                'image': args.repo,
+                'digest': digest
+            }
             buildmeta.write(artifact_name=args.metajsonname)