Update GitLab.

docker-compose.yml

@@ -2,7 +2,7 @@
 version: '3.7'
 services:
   gitlab:
-    image: registry.hub.docker.com/gitlab/gitlab-ce:15.5.4-ce.0
+    image: registry.hub.docker.com/gitlab/gitlab-ce:15.6.0-ce.0
     restart: always
     environment:
       # yamllint disable rule:line-length