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

templates/docker-compose.nextcloud.yml.j2

@@ -25,12 +25,21 @@ services:
     container_name: nextcloud-app
     depends_on:
       - mysqldb
+      - redis
     volumes:
       - nextcloud:/var/www/html
       - /etc/localtime:/etc/localtime:ro
     env_file:
       - nextcloud.env
 
+  redis:
+    image: bitnami/redis:latest
+    restart: unless-stopped
+    volumes:
+      - redis:/data
+    env_file:
+      - redis.env
+
   web:
     image: nginx
     restart: always
@@ -85,6 +94,7 @@ services:
 
 volumes:
   mysqldb:
+  redis:
   nextcloud:
 {% if nextcloud_enable_restic_compose_backup %}
   backup-cache: