GitHub Actions has support for reporting an error on a file, so it can be seen directly in changed files.
This will make it even quicker to find where the merge conflict was accidentally merged in.

https://help.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-an-error-message

Specifically creyD/prettier_action. As far as I can tell the main difference is using JS and not docker/shell. Yours runs faster but his seems to be more full-featured. I think that adding a comparison to your readme would be worthwhile to show your advantages/disadvantages.