chore(deps): update docker.io/library/postgres docker tag to v18

Author: renovate-bot-cbcoutinho[bot]

docker-compose.yml

@@ -394,7 +394,7 @@ services:
   # Bring up with:  docker compose --profile postgres up -d postgres-test
   # See ADR-026 for the pluggable-database-backend design.
   postgres-test:
-    image: docker.io/library/postgres:16-alpine@sha256:e013e867e712fec275706a6c51c966f0bb0c93cfa8f51000f85a15f9865a28cb
+    image: docker.io/library/postgres:18-alpine@sha256:1b1689b20d16a014a3d195653381cf2caa75a41a92d93b255a9d6ea29fd353aa
     restart: unless-stopped
     environment:
       POSTGRES_USER: mcp