Use multi-stage build for Docker image

1. Allows to build the composer dependencies with official composer img
2. php-fpm --nodaemonize: Force to stay in foreground and ignore daemo-
   nize option from configuration file

nginx.conf

@@ -24,7 +24,7 @@ http {
     server {
         listen       80 default_server;
 
-        root /var/www/public;
+        root /app/public;
         index index.php;
         charset utf-8;