How are you running WordPress? The image variant you’re using matters here.
If you’re running a variant that has a web server built in (like Apache) then a reverse proxy will work fine.
If you’re running the variant that uses php-fpm, then you’ll need to use php_fastcgi, and for that to work, Caddy will need to share a volume with the WordPress container’s webroot so that Caddy can read the PHP files to execute.