Add an mu-plugin to fix automatic security updates

<?php

/**

 * Plugin Name: File Modification Blocker

 * Plugin URI: https://wordpress.stackexchange.com/a/300718

 * Description: Block file modification except for security updates.

 * Licence: CC BY-SA 3.0

 * Licence URI: https://creativecommons.org/licenses/by-sa/3.0/

 * Author: Marc Guay

 * Author URI: https://wordpress.stackexchange.com/users/69982/marcguay

 */

add_filter( 'file_mod_allowed', 'override_file_mod_allowed', 10, 2 );

function override_file_mod_allowed( $setting, $context ) {

	if ( $context == 'automatic_updater' ) {

		return true;

	}

	return $setting;

}