set DB port to config.php.sample (#74)

With this fix we can use DB_PORT variable

rootfs/flarum/app/config.php.sample

@@ -3,6 +3,7 @@
   'database' => array (
     'driver' => 'mysql',
     'host' => '<DB_HOST>',
+    'port' => '<DB_PORT>',
     'database' => '<DB_NAME>',
     'username' => '<DB_USER>',
     'password' => '<DB_PASS>',

rootfs/usr/local/bin/startup

@@ -84,6 +84,7 @@ if [ -e '/flarum/app/public/assets/rev-manifest.json' ]; then
          -e "s|<DB_USER>|${DB_USER}|g" \
          -e "s|<DB_PASS>|${DB_PASS}|g" \
          -e "s|<DB_PREF>|${DB_PREF}|g" \
+         -e "s|<DB_PORT>|${DB_PORT}|g" \
          -e "s|<FORUM_URL>|${FORUM_URL}|g" /flarum/app/config.php.sample
 
   cp -p /flarum/app/config.php.sample /flarum/app/config.php