Update docker.io/library/mariadb Docker tag to v12

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

@@ -3,13 +3,13 @@
 
 services:
   database:
-    image: docker.io/library/postgres:18-alpine
+    image: docker.io/library/postgres:15-alpine
     environment:
       - "POSTGRES_USER=hedgedoc"
       - "POSTGRES_PASSWORD={{ secret__hedgedoc_db_password }}"
       - "POSTGRES_DB=hedgedoc"
     volumes:
-      - database:/var/lib/postgresql
+      - database:/var/lib/postgresql/data
     restart: unless-stopped
 
   app: