Update docs regarding tags in custom cache image name

README.md

@@ -62,7 +62,7 @@ or
 
 - **pull_image_and_stages**: Set to `false` to avoid pulling from the registry or to build from scratch (default: `true`).
 
-- **stages_image_name**: Set custom name for stages. Useful if using a job matrix (default: `$image_name-stages)`.
+- **stages_image_name**: Set custom name for stages/cache. Useful if using a job matrix (default: `${image_name}-stages`). Do not use tags here to avoid confusion (tags are ignored in new versions) or errors (in old versions).
 
 - **push_image_and_stages**: Test a command before pushing. Use `false` to not push at all (default: `true`).