Update docker.io/library/redis Docker tag to v8

resources/chaosknoten/tickets/docker_compose/compose.yaml.j2

@@ -1,13 +1,13 @@
 ---
 services:
   database:
-    image: docker.io/library/postgres:18-alpine
+    image: docker.io/library/postgres:15-alpine
     environment:
       - "POSTGRES_USER=pretix"
       - "POSTGRES_PASSWORD={{ secret__pretix_db_password }}"
       - "POSTGRES_DB=pretix"
     volumes:
-      - database:/var/lib/postgresql
+      - database:/var/lib/postgresql/data
     networks:
       backend:
     restart: unless-stopped