Rebuild the Docker image on Alpine: ~735MB -> ~89MB

Stage 2 was php:8.3-apache (Debian), which compiles PHP from source
with --with-apxs2 for mod_php -- that base image alone is 719MB of
our 735MB, before any app code. Replaced with alpine:3.24 + apk's own
prebuilt php83/php83-apache2/apache2 packages: same architecture (one
process, mod_php, .htaccess-driven rewriting), no fpm/nginx rewrite
needed.

- docker/apache.conf: rewritten for Alpine's apache2 (mod_rewrite ships
  but isn't loaded by default; a different default document root/log
  paths). Logs redirected to stdout/stderr so `docker logs` still shows
  them -- Alpine's own defaults write to a real file under ServerRoot,
  unlike the official Debian image's symlinked paths.
- docker/entrypoint.sh: su-exec instead of su -- BusyBox's su doesn't
  take the same -c/user argument order as the GNU one the previous
  entrypoint relied on. Also moved earlier in the Dockerfile (with the
  other rarely-changing setup, before COPY . .) so it no longer re-runs
  on every build for a file that essentially never changes.
- Composer's binary is still borrowed from the official composer:2
  image via multi-stage COPY, not apk's own `composer` package, which
  turned out to pull in an entire second PHP interpreter (php85) as a
  dependency just to run itself.
- ext-iconv needed adding explicitly (symfony/polyfill-mbstring depends
  on it; the official Debian image bundles it by default, apk doesn't).

Verified against the real compose stack, not just that it builds: apk
install; composer install; migrations on startup; PHPUnit 88/88 (runs
on the host, but confirms nothing else broke); and by hand, all
through the actual container -- health check, SPA fallback for unknown
routes, static assets served directly, the API's 401 guard, and a full
magic-link -> verify -> JWT -> authenticated project create/list round
trip via the real Mailpit catcher.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/setup.md

@@ -13,7 +13,9 @@ docker compose up -d
 ```
 
 This runs a multi-stage build — a Node stage compiles the Vue frontend, then a
-PHP 8.3 + Apache stage bakes in the PHP source and the built SPA — applies
+PHP 8.3 + Apache stage (Alpine-based; apk's own prebuilt packages rather than
+compiling PHP from source, which is most of why the image is ~90MB rather
+than several times that) bakes in the PHP source and the built SPA — applies
 migrations, and serves the whole app at <http://localhost:8080>: the SPA at `/`
 (assets and all) and the REST API under `/api` (e.g.
 `curl http://localhost:8080/api/health`). Unknown paths fall back to the SPA