chore: update PG_VERSION from 17 to 18 in CI workflow

[pyramation]

chore: update PG_VERSION from 17 to 18 in CI workflow

Summary

The CI workflow (.github/workflows/docker.yml) was still hardcoded to build and publish PostgreSQL 17 images, even though the Dockerfile already defaults to PG_VERSION=18. This one-line change updates the workflow's PG_VERSION env var from '17' to '18' so that published images use PostgreSQL 18.

This affects:

• The PG_VERSION build-arg passed to docker build
• The image tag (images will now be tagged 18 and latest on pushes to main)

Review & Testing Checklist for Human

• Verify the postgres:18-alpine base image exists on Docker Hub and is stable enough for your use case
• Confirm the bundled extensions (pgvector 0.8.2, PostGIS 3.6.2, pg_textsearch 0.6.1) build successfully against PG 18 — CI on this PR should validate this via the build step
• After merge, check that the published image at ghcr.io/constructive-io/docker/postgres-plus:18 works correctly (pull and run CREATE EXTENSION vector; CREATE EXTENSION postgis; CREATE EXTENSION pg_textsearch;)

Notes

• Requested by: @pyramation
• Devin Session

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✅ Devin Review: No Issues Found

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