🔧 Enable automatic docker image prune again

The feature was part of the old role but unconditionally.  When porting
to external role we made it optional with variable
`docker_cron_image_prune` but default to false.  Restore the previous
behaviour in this playbook.

group_vars/docker_host/vars.yml

@@ -1,2 +1,3 @@
 ---
 docker_data_root: "/srv/docker"
+docker_cron_image_prune: true

host-wittgenstein.yml

@@ -6,6 +6,7 @@
     - role: netz39.host_docker
       vars:
         docker_data_root: "/srv/docker"
+        docker_cron_image_prune: true
     - role: apache
     - role: apache_letsencrypt  # Uses configuration from dehydrated setup
     - role: 24367dfa.dehydrated