fix(docker): commit composer.lock and install intl extension

- Remove composer.lock from .gitignore so it is tracked and included in
  the build context; without it composer install resolves fresh versions
  on every CI build, causing cache/vendor mismatches that produce the
  RewindableGenerator and LazyGhostTrait runtime errors in prod
- Install the intl PHP extension in the base stage to remove the
  Symfony startup deprecation warning; icu-dev and libzip-dev are only
  kept for the compile step, then deleted to keep the layer lean

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -3,5 +3,4 @@
 /vendor/
 /var/
 /public/bundles/
-composer.lock
 /.phpunit.cache

Dockerfile

@@ -1,9 +1,9 @@
 FROM dunglas/frankenphp:1-php8.4-alpine AS base
 
-RUN apk add --no-cache \
+RUN apk add --no-cache icu-dev libzip-dev \
-    curl \
+    && docker-php-ext-install -j$(nproc) intl \
-    icu-libs \
+    && apk del icu-dev libzip-dev \
-    libzip
+    && apk add --no-cache curl icu-libs libzip
 
 WORKDIR /app