Issue with PostgreSQL 18.0 Docker image volume mount (/var/lib/postgresql/data symlink)

Hello,

I’ve encountered a problem when trying to upgrade from the `postgres:17.6` Docker image to `postgres:18.0`.

With PostgreSQL 17.6, the volume mapping worked fine:

```yaml
volumes:
  - /volumeUSB1/usbshare1-6/configs/postgresql/data:/var/lib/postgresql/data/
```

However, with `postgres:18.0`, the container fails to start with the following error:

```
Error response from daemon: failed to create task for container: failed to create shim task:
OCI runtime create failed: runc create failed: unable to start container process:
error during container init: error mounting "/volumeUSB1/usbshare1-6/configs/postgresql/data"
to rootfs at "/var/lib/postgresql/data": change mount propagation through procfd:
open o_path procfd: open
/volumeUSB1/usbshare1-6/docker/btrfs/subvolumes/362b5618ea6ff38caa0671921e892066b3ad705e39270f938aac396f7401781e/var/lib/postgresql/data:

no such file or directory: unknown
```

I noticed that the difference is in the Docker image itself:

* In 17.6, `/var/lib/postgresql/data` is a real directory.
* In 18.0, `/var/lib/postgresql/data` is a symlink to `.`.


This change seems to break bind mounts on some storage backends (in my case, Btrfs on a Synology NAS).

Could you confirm if this symlink change in the official PostgreSQL 18.0 Docker image is intentional?
And if so, is there a recommended way to mount external volumes for `PGDATA` now?

Thanks,
Pavel Hushcha