Docker: Fix PHP tests

This creates another mysql_testing database during db service setup

Replace server with env tags in phpunit.xml in order to force
override certain parameters when tests are run. See:
https://github.com/sebastianbergmann/phpunit/issues/2353 for more
information.

Rename primary developer Docker database from bookstack-test to
bookstack-dev. bookstack-test is used as the mysql_testing database

app/Config/database.php

@@ -81,10 +81,11 @@ return [
         'mysql_testing' => [
             'driver'    => 'mysql',
             'url' => env('TEST_DATABASE_URL'),
-            'host'      => '127.0.0.1',
+            'host'      => $mysql_host,
             'database'  => 'bookstack-test',
             'username'  => env('MYSQL_USER', 'bookstack-test'),
             'password'  => env('MYSQL_PASSWORD', 'bookstack-test'),
+            'port'      => $mysql_port,
             'charset'   => 'utf8mb4',
             'collation' => 'utf8mb4_unicode_ci',
             'prefix'    => '',