fix: updated postgre volume mount point path

[viadezo1er]

Postgre 18 has moved the volume mount point from /var/lib/postgresql/data to /var/lib/postgresql
Fixes #122

See line 194 of the Dockerfile of postgres:18-alpine:
https://github.com/docker-library/postgres/blob/22ca5c8d8e4b37bece4d38dbce1a060583b5308a/18/alpine3.22/Dockerfile#L194
NOTE: in 18+, PGDATA has changed to match the pg_ctlcluster standard directory structure, and the VOLUME has moved from /var/lib/postgresql/data to /var/lib/postgresql

So to prevent a useless volume being created for the postgre docker each time the container is started, accordingly update the mount point of the postgre volume.

After reading the Contributor Guidelines for etherpad-lite:

• there isn't a develop branch for etherpad-go
• it's a bug fix but I'm not sure a regression test is really needed here (I think they are aimed at Typescript/Go bugs, not docker bugs)
• I have no idea how to run a go test suite, and it's probably irrelevant here anyway (works locally and on a VPS, data is persistent)
• No documentation update needed

[viadezo1er]

Sorry for the double PR (the other is closed), I incorrectly assumed the issue would be number 68.

[viadezo1er]

At some point in the future, etherpad-lite might update from postgres:15-alpine to postgres:18-alpine or later, would it be relevant to add a warning in the docker-compose.yml in etherpad-lite repo?