refactor: Remove unnecessary variable

nextcloud/docker-compose.yml

@@ -59,7 +59,7 @@ services:
 
   nextcloud-postgres:
     container_name: ${POSTGRES_CONTAINER_NAME}
-    image: ${POSTGRES_IMAGE}
+    image: ${POSTGRES_IMAGE:-postgres:12.7-alpine}
     restart: always
     environment:
       POSTGRES_USER: ${POSTGRES_USER}