Add upgrade note about PGDATA data loss bug fixed in 2.55.4

PostgreSQL 18 changed its default PGDATA path, causing data to be
written to the container's ephemeral layer instead of the Docker volume.

Author: valentijnscholten

docs/content/releases/os_upgrading/2.51.md

@@ -77,6 +77,14 @@ If you already have a valid backup of the postgres 16 database, you can start at
 
 _Note: If you are using a bound volume, the path has changed for Postgres18. It is now `/var/lib/postgresql/` instead of `/var/lib/postgresql/data`. Failure to change the path may result in errors about failure to create a shim task. See the discussion in [docker-library/postgres](https://github.com/docker-library/postgres/issues/1370)._
 
+> **Warning: Data loss after `docker compose down` (fixed in 2.55.4)**
+>
+> PostgreSQL 18 changed its default `PGDATA` path from `/var/lib/postgresql/data` to `/var/lib/postgresql/18/docker`. Because the Docker volume was mounted at `/var/lib/postgresql/data`, data was written to the container's ephemeral layer instead of the volume.
+>
+> This has been fixed in 2.55.4 by explicitly setting `PGDATA: /var/lib/postgresql/data` in `docker-compose.yml`.
+>
+> If you customise the postgres service in your own `docker-compose.override.yml`, make sure `PGDATA` is set to the path where your volume is mounted.
+
 ### 0. Backup
 
 Always back up your data before starting and save it somewhere.