fix(docker): move Postgres to a named volume and bump to Postgres 18 (beta)

[therealbrad]

Description

Ports the fix from #486 (targeting main) to beta. Fixes #485: docker compose build prod fails at the [internal] load build context step with a permission error on docker-data/postgres.

Root cause: the compose files bind-mount ./docker-data/postgres directly onto /var/lib/postgresql/data. Once Postgres has run, it locks that directory down to 0700. The app builds use context: .. (the repo root), so that directory sits inside the build context. BuildKit's context sender needs to stat/read every entry under the context root — a permission-denied directory there aborts the whole context transfer, regardless of .dockerignore rules, since the walk fails before exclusion patterns are applied. Verified empirically on #486 (see that PR for the full repro/verification writeup); this PR applies the same verified fix to the files tracked on beta.

Changes

• Postgres now uses a named Docker volume (testplanit-postgres-data) instead of a bind mount, in both compose files tracked on this branch (docker-compose.yml, docker-compose.dev.yml).
• Fixed the .dockerignore inconsistency across all three ignore files (hygiene; verified insufficient alone for this bug in fix(docker): move Postgres to a named volume and bump to Postgres 18 #486).
• Bumped Postgres 15 → 18 (three majors behind current stable) while in here. Pinned PGDATA=/var/lib/postgresql/data since the 18+ image defaults to a version-specific subdirectory and expects the volume mounted at /var/lib/postgresql instead of .../data (docker-library/postgres#1259) — reproduced locally — and the new layout has a still-open detection bug (docker-library/postgres#1400) that also reproduced reliably in testing.
• Docs updated (docker-setup.md, deployment.md) with the new data-persistence layout, backup/restore commands, and a migration note for existing deployments (Postgres 18 can't read a Postgres 15 data directory, so this needs a real pg_dump/pg_restore, not a raw file copy).

Note on docker-compose.prod.yml

That file isn't tracked on beta — it was removed in db5cfd83a ("remove prod compose from tracking; add to gitignore") because it contains server-specific IPs, and isn't touched by this PR. Whoever maintains that file's local/server copy needs the equivalent change (named volume + PGDATA pin + Postgres 18 tag) applied manually.

Related Issue

Relates to #485 (fixed on main via #486; this ports the same fix to beta)

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)

Breaking in the sense that existing deployments must run the documented pg_dump/pg_restore migration before their next docker compose build/up.

How Has This Been Tested?

• Manual testing

Same verification approach as #486, re-run against this branch's actual files:

• Brought up postgres from docker-compose.yml (dev profile) with the named-volume override — reports healthy, SELECT version() confirms PostgreSQL 18.4.
• Ran a full down / up cycle — a marker row written before the restart survives after, confirming the named volume persists data correctly across container recreation.
• Validated docker compose config resolves cleanly for both tracked compose files.
• Content is otherwise identical to what was Prettier-checked and build/integration-tested on main in fix(docker): move Postgres to a named volume and bump to Postgres 18 #486 (this worktree had no local node_modules to re-run Prettier against, but the patch applied via git apply --check with zero fuzz, confirming byte-identical content).

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have signed the CLA

Additional Notes

None of this touches application source, tests, or types.