Local environment setup cleanups (follow-ups to #1752)

[pkevan]

Follow-ups to #1752, batching the safe local-environment setup cleanups from a review of .docker/. Opened as a draft for review.

Changes

1. One-command host setup (package.json, .docker/readme.md)
package.json had no scripts block. Added:

• npm run setup → composer install + npm ci + npm run build --workspaces --if-present
• npm run build → the workspace build alone

The readme's separate "composer install" and "npm install / npm run build" steps collapse into a single npm run setup. (npm ci replaces npm install for reproducible installs against the committed lockfile.)

2. Non-destructive git hooks (.docker/readme.md)
rm -rf .git/hooks && ln -s .githooks .git/hooks → git config core.hooksPath .githooks (git ≥ 2.9). One command, doesn't delete the hooks dir.

3. PHPUnit test suite auto-installs (.docker/Dockerfile.phpunit, new .docker/bin/install-test-suite.sh, .docker/readme.md)

• New entrypoint waits for the DB, then installs the WP test suite on first run (idempotent; resilient — a failed download warns and still starts the container rather than crashing it). Removes the manual "exec in and run install-wp-tests.sh" step.
• Also fixes a version drift: the container pinned yoast/phpunit-polyfills:^1.0 while composer.json requires ^4.0 — now aligned.

4. Single Docker boot command (.docker/readme.md)
docker compose build --pull + docker compose up → docker compose up --build.

6. Dockerfile.php-fpm image hygiene (no behavior change)
Same packages, extensions, and config — just consolidated the ~18 layers into a handful, added --no-install-recommends + apt-get clean + rm -rf /var/lib/apt/lists/*, and &&-chained the wkhtmltopdf / WP-CLI / php.ini steps so intermediate artifacts (the wkhtmltox tarball) don't bloat a layer. Mirrors the cleanup pattern Dockerfile.phpunit already uses.

Item 5 from the review (clone WP trunk instead of a pinned branch) is in #1752, so this branch deliberately leaves readme.md step 4 untouched to avoid a conflict.

Verification

• package.json is valid JSON; npm run setup/build resolve.
• bash -n clean on the new entrypoint script.
• docker build --check clean on both Dockerfiles.
• Full docker build of both images succeeds (exit 0). The php-fpm image smoke-tests clean: WP-CLI 2.12.0, gd/intl/mysqli/zip/opcache loaded, memory_limit = 256M, msmtp + mailcatcher present. The phpunit image installs phpunit/phpunit 9.6.34 + yoast/phpunit-polyfills 4.0.0 (drift fixed).
• The PHPUnit auto-install entrypoint's runtime path (DB wait + first-run install) wasn't exercised end-to-end here — worth a local docker compose -f docker-compose.phpunit.yml up before merging.