Warning: include_once(/app/vendor/autoload.php): Failed to open stream: No such file or directory in /app/index.php on line 3

Warning: include_once(): Failed opening '/app/vendor/autoload.php' for inclusion (include_path='.:/usr/local/lib/php') in /app/index.php on line 3
Couldn't include '/app/vendor/autoload.php'. Did you run `composer install`?