Set tag list input's required field from true to false (SOFTWARE-4832)

Author: Jack Yuan

action.yml

@@ -8,7 +8,7 @@ inputs:
     required: true
   tag_list:
     description: 'List of comma-separated tags that gets passed along to the docker build args'
-    required: true
+    required: false
     default: '' # set to an empty string
   registry_url:
     description: 'URL of the registry'