feat: officially support postgres 18, add docs for volume mount change

[swiffer]

this should be merged shortly after postgres 18 is officially released.

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[parkr]

It looks like for v17 folks can opt-in to this behavior with:

environment:
  PGDATA=/var/lib/postgresql/17/docker
database:
  volumes:
    - teslamate-db:/var/lib/postgresql

I'm on v16 so I'll try upgrading to v17 with this configuration.

[swiffer]

@JakobLichterfeld / @parkr - Postgres 18 has been released on 2025-09-25. This PR has been updated and is ready to be reviewed / merged.

[JakobLichterfeld]

Thanks so much!

[JakobLichterfeld]

It will be merged when the next release happens. As the documentation is automatically updated, otherwise users will end up using PostgreSQL 18, but the TeslaMate version does not yet allow it.