I like the concept, you could also detect the compose file name automatically.

Or write down hashes for tag even if compose don’t use your variables so reading from compose, so people would not need to relay on the script but could use it as additional backup

Check also all parent directories, as the whole path need to have access (execute) permissions.

Sounds like a permission problem, probably when you did copy the files then permission did change from like www-data into your user, simplest solution would be to change the permissions for those files (for example to 777 as a temporary solution)