Mount postgres volumes at /var/lib/postgresql

[mattgallagher92]

This is to resolve errors like the following, now that postgres:latest resolves to the v18 image by default.

in 18+, these Docker images are configured to store database data in a
format which is compatible with "pg_ctlcluster" (specifically, using
major-version-specific directory names).  This better reflects how
PostgreSQL itself works, and how upgrades are to be performed.

See also https://github.com/docker-library/postgres/pull/1259

Counter to that, there appears to be PostgreSQL data in:
  /var/lib/postgresql/data (unused mount/volume)

This is usually the result of upgrading the Docker image without
upgrading the underlying database using "pg_upgrade" (which requires both
versions).

The suggested container configuration for 18+ is to place a single mount
at /var/lib/postgresql which will then place PostgreSQL data in a
subdirectory, allowing usage of "pg_upgrade --link" without mount point
boundary issues.

See https://github.com/docker-library/postgres/issues/37 for a (long)
discussion around this process, and suggestions for how to do so.

[mattgallagher92]

Note: I'm not a Postgres expert, so this configuration may be undesirable for reasons that I haven't considered. Of course, feel free to edit/reject this PR if that's the case.

[stevensJourney]

Thanks for this! For our simple demo use-case, this looks good to me. I'll make a minor followup PR to add a changelog entry for this. I'll also explicitly set the Postgres Docker image version there.