Update docker.io/library/postgres Docker tag to v18

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

@@ -46,7 +46,7 @@ services:
       - "8080:8080"
 
   db:
-    image: docker.io/library/postgres:18.0@sha256:41fc5342eefba6cc2ccda736aaf034bbbb7c3df0fdb81516eba1ba33f360162c
+    image: docker.io/library/postgres:18.0@sha256:6f3e42ad37decc037c508dd405f5941c6dfa7d21f21a7237a43bde9def1e295e
     restart: unless-stopped
     networks:
       - keycloak