Adds Redis cache (#4)

Requires adding a Redis host password through `redis_host_password`.

If you are updating you existing Nextcloud, you might have to manually update the `<nextcloud-data-volume>/config/config.php` and replace `memcache...` properties with:

```
  'memcache.distributed' => '\OC\Memcache\Redis',
  'memcache.local' => '\OC\Memcache\Redis',
  'memcache.locking' => '\OC\Memcache\Redis',
  'redis' => array(
     'host' => 'redis',
     'port' => 6379,
     ),
```

Co-authored-by: Jan Beilicke <dev@jotbe.io>
Reviewed-on: #4

tasks/main.yml

@@ -69,6 +69,17 @@
       SMTP_PASSWORD={{ smtp_password }}
       MAIL_FROM_ADDRESS={{ mail_from_address }}
       MAIL_DOMAIN={{ mail_domain }}
+      REDIS_HOST=redis
+      REDIS_HOST_PASSWORD={{ redis_host_password }}
+
+- name: Provide Redis env vars
+  copy:
+    dest: /home/{{ docker_user }}/nextcloud/redis.env
+    owner: "{{ docker_user }}"
+    group: "{{ docker_user }}"
+    mode: '0640'
+    content: |
+      REDIS_PASSWORD={{ redis_host_password }}
 
 - name: Provide restic-compose-backup env vars
   copy: