The scenarios worked around Postgres 18's data layout change by mounting the
tmpfs at /var/lib/postgresql. The postgres role solved the same problem
differently and better, when it bit us there: it sets PGDATA explicitly, and to
/data rather than anywhere beneath /var/lib/postgresql, because that path is a
VOLUME in the official image and nothing can be mounted under it.

Doing the same here means the scenarios exercise the layout the role actually
deploys, and that a future image changing its own default cannot move the data
directory out from under them.

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

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT