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@sha256:1ffc019dae94eca6b09a49ca67d37398951346de3c3d0cfe23d8d4ca33da83fb
+    image: docker.io/library/postgres:18.0@sha256:b3ef52d269e0050ac3ba8266d8bc9650987412f60d66472fdf7e14f523ecc506
     restart: unless-stopped
     networks:
       - keycloak