Add redis

This isn't really used as a cache, but it is for a couple bits, so nice to enable it anyway, and it might become so in future

ansible/roles/pve_docker/files/synapse/docker-compose.yml

@@ -32,6 +32,12 @@ services:
     volumes:
       - /mnt/tank/dbs/postgres/synapse:/var/lib/postgresql/data
 
+  redis:
+    image: redis:6-alpine
+    restart: unless-stopped
+    volumes:
+      - /mnt/tank/dbs/redis/synapse:/data
+
   admin:
     image: awesometechnologies/synapse-admin:latest
     restart: unless-stopped