Guarantee REDIS_HOST_PORT is never null for UNIX sockets

Addresses #1785 (which arises when using UNIX sockets)

Without this fix (or manually specifying a bogus REDIS_HOST_PORT) UNIX socket connections won't work if REDIS_HOST is used to specify the socket to the container. 

Signed-off-by: Josh R. <josh.t.richards@gmail.com>

.config/redis.config.php

@@ -13,5 +13,7 @@ if (getenv('REDIS_HOST')) {
     $CONFIG['redis']['port'] = (int) getenv('REDIS_HOST_PORT');
   } elseif (getenv('REDIS_HOST')[0] != '/') {
     $CONFIG['redis']['port'] = 6379;
+  } elseif (getenv('REDIS_HOST')[0] == '/') {
+    $CONFIG['redis']['port'] = 0;
   }
 }