docker-compose.yml: PostgreSQL mount layout

[jpwiedekopf]

The layout for PostgreSQL has changed in version 18, leading to the container terminating with the following error:

Error: 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.

The easy fix is changing the volume definition in the docker-compose.yml for the db container to target /var/lib/postgresql instead of /var/lib/postgresql/data.

[schwzr]

Good catch!

One could say that we would like users to follow the instructions at https://dsf.dev/operations/latest/upgrade-from-1.html#upgrade-postgresql-from-15-to-18 ;-)

But seriously: we will fix that later!