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?

[SamTV12345]

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?

Thanks for the pr. Yes this is an issue. Thanks for raising:

Important Note: (for PostgreSQL 17 and below) Mount the data volume at /var/lib/postgresql/data and not at /var/lib/postgresql because mounts at the latter path WILL NOT PERSIST database data when the container is re-created. The Dockerfile that builds the image declares a volume at /var/lib/postgresql/data and if no data volume is mounted at that path then the container runtime will automatically create an [anonymous volume](https://docs.docker.com/engine/storage/#volumes) that is not reused across container re-creations. Data will be written to the anonymous volume rather than your intended data volume and won't persist when the container is deleted and re-created.

[SamTV12345]

You can run the tests with e.g. Docker Desktop installed. Just by doing go test ./... Pretty cool that everything is integrated in the toolchain