fix pihole upgrade_strategy (#1810)

library/ix-dev/charts/pihole/upgrade_strategy

@@ -11,7 +11,7 @@ RE_STABLE_VERSION = re.compile(r'\d+\.\d+\.\d+')
 
 def newer_mapping(image_tags):
     key = list(image_tags.keys())[0]
-    tags = {t for t in image_tags[key] if RE_STABLE_VERSION.fullmatch(t)}
+    tags = {t: t for t in image_tags[key] if RE_STABLE_VERSION.fullmatch(t)}
     version = semantic_versioning(list(tags))
     if not version:
         return {}