Update docker.io/pretix/standalone Docker tag to v2026

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

@@ -46,12 +46,12 @@ services:
       - "8080:8080"
 
   db:
-    image: docker.io/library/postgres:18
+    image: docker.io/library/postgres:15.17
     restart: unless-stopped
     networks:
       - keycloak
     volumes:
-      - "./database:/var/lib/postgresql"
+      - "./database:/var/lib/postgresql/data"
     environment:
       POSTGRES_USER: keycloak
       POSTGRES_PASSWORD: {{ secret__keycloak_db_password }}