Postgres 18

[s01ipsist]

Adds support for Postgres 18 by adding conditional logic to set the data volume path differently for before/after v 18, as guided by https://hub.docker.com/_/postgres

The defined VOLUME was changed in 18 and above to /var/lib/postgresql. Mounts and volumes should be targeted at the updated location

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

Fixes #364
Replaces #358 / #367

Not sure if this needs to be fixed or not, but there's a subtle functional change in the result with this code as is.

• v17 service stays running after creation
• v18 service stops after creation, and requires a separate call to dokku postgres:start to re-start service
  This appears to be because of the logic in
  https://github.com/docker-library/postgres/blob/master/docker-entrypoint.sh#L360
  v17 triggers the path with message output

PostgreSQL Database directory appears to contain a database; Skipping initialization

v18 triggers the path with message output

PostgreSQL init process complete; ready for start up.

and stops the container.