This allows for a more graceful error message for the case of users who need to update their volume path.
I tested this with 17 and `--env PGDATA=/var/lib/postgresql/17/docker` (to make sure it still works fine and doesn't complain about the extra volume provided by Docker/`VOLUME`) and 18 with `-v /var/lib/postgresql/data` (where it appropriately errors and explains to me what I should be doing instead).
I also did a reflow on some text in the error message that's designed to fit in 80 columns but was taking 81. 😂
* Unset NOTIFY_SOCKET when running the temporary server.
Postgres has native support for this notification socket and will write a `READY=1` message once it is ready and accepting connections. Unfortunately, the temporary server used by the `docker-entrypoint.sh` also sends a message on the socket, making it appear as though the container is ready and serving connections when it is not.
