cmd/push-container-manifest: use additional-images rather than overwrite

in coreos/fedora-coreos-pipeline#1091 we started
publishing container images in multiples repos.
However the build.json was not reflecting that, only storing the last
pushed image data, overwriting it with each arch.

See #4038
See coreos/fedora-coreos-pipeline#1099

Author: jbtrystram

src/cmd-push-container-manifest

@@ -122,11 +122,15 @@ def main():
             arch = manifest['platform']['architecture']
             if arch in map_arch:
                 arch = map_arch[arch]
-            buildmetas[arch][args.metajsonname] = {
-                'image': args.repo,
-                'digest': manifest['digest'],
-                'tags': args.tags
-            }
+            image = {
+                    'image': args.repo,
+                    'digest': manifest['digest'],
+                    'tags': args.tags
+                }
+            if buildmetas[arch].get(args.metajsonname, 'null') != 'null':
+                buildmetas[arch][args.metajsonname].additional_images += [image]
+            else:
+                buildmetas[arch][args.metajsonname] = image
             buildmetas[arch].write(artifact_name=args.metajsonname)