refs #12464-24

* switch docker image vufind2 from php54 to php_5.6

docker-compose.yml

 version: '2'
 services:
   lamp:
-    image: docker.io/ubleipzig/vufind2:php54
+    image: docker.io/ubleipzig/vufind2:php_5.6
     ports:
       - 80:80
       - 443:443