chore(deps): update postgres docker tag to v18

Author: renovate[bot]

backend/docker-compose.yml

@@ -20,7 +20,7 @@ services:
             - "8080:8080"
 
     database:
-        image: postgres:14-alpine@sha256:6765739f422606933bc2aece3a2288e40e491488fd7e7c14e3323dfeefb10e38
+        image: postgres:18-alpine@sha256:96d56f7f57c6aacd1fcb908bc83b345ec5f83231ee486dd66a1baadce274db88
         environment:
             - POSTGRES_USER=postgres
             - POSTGRES_PASSWORD=${TORMAP_DATABASE_PASSWORD:?Set TORMAP_DATABASE_PASSWORD to a strong generated password}