chore(deps): update postgres docker tag to v18

Author: renovate[bot]

linkwarden/compose.yml

@@ -27,7 +27,7 @@ services:
       - traefik.http.routers.linkwarden.tls.certresolver=cloudflare
   linkwarden-database:
     container_name: linkwarden-database
-    image: postgres:17-alpine
+    image: postgres:18-alpine
     restart: unless-stopped
     environment:
       - POSTGRES_PASSWORD=${LINKWARDEN_POSTGRES_PASSWORD}