[Docker] replace curl healtcheck with php/wget

Curl pulls in quite some dependencies we don't really need. wget or pure php is way lighter on that.
Pure php could work like this:
````
php -r 'if(!file_get_contents(\"http://localhost/api/fever.php\")){exit(1);}'
````