Separate the PHP versions to multiple dockerfiles

Because there's some difference in installing some libraries
between PHP 5 and PHP 7. There must be multiple php dockerfiles.
For that I created 3 docker files each of a different version,
this will make it easy for the users to switch between the PHP
version by just editing the docker-compose file and not touching
the dockerfiles.

docker-compose.yml

@@ -18,7 +18,9 @@ services:
 ### PHP Container ###########################################
 
     php:
-        build: ./php
+        build:
+            context: ./php
+            dockerfile: Dockerfile-php-70
         volumes:
             - ../:/var/www/laravel
             - ./logs/php/:/usr/local/var/log