Add mysql 5.7 option

Author: Gareth Midwood

src/Environment/Command/Container/Mysql.php

@@ -50,13 +50,14 @@ protected function askQuestions()
 
         $this->buildOrImage(
             '../vendor/creode/docker/images/mysql',
-            'creode/mysql:5.6',
+            'creode/mysql:5.7',
             $this->_config,
             [   // builds
                 '../vendor/creode/docker/images/mysql' => 'MySQL'
             ],
             [   // images
-                'creode/mysql:5.6' => 'MySQL'
+                'creode/mysql:5.6' => 'MySQL 5.6',
+                'creode/mysql:5.7' => 'MySQL 5.7'
             ]
         );