docker/php: build extensions via mlocati/php-extension-installer

Hand-managed apk build deps + docker-php-ext-install was failing to build
on the server. Switch to the extension-installer image (same approach the
escapepage stack uses), which resolves system libs itself.

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

docker/php/Dockerfile

@@ -1,20 +1,16 @@
 FROM php:8.5-fpm-alpine
 
-RUN apk add --no-cache \
-        bash \
-        git \
-        icu-dev \
-        libzip-dev \
-        zip \
-        unzip \
-        $PHPIZE_DEPS \
-    && docker-php-ext-configure intl \
-    && docker-php-ext-install -j"$(nproc)" \
-        intl \
-        pdo_mysql \
-        opcache \
-        zip \
-    && apk del $PHPIZE_DEPS
+RUN apk add --no-cache bash git unzip
+
+# Install PHP extensions with mlocati/php-extension-installer: it pulls the
+# right system libraries itself, builds, and cleans up - much less fragile
+# than hand-maintaining apk build deps.
+COPY --from=mlocati/php-extension-installer:latest /usr/bin/install-php-extensions /usr/local/bin/
+RUN install-php-extensions \
+    intl \
+    pdo_mysql \
+    opcache \
+    zip
 
 COPY --from=composer:2 /usr/bin/composer /usr/bin/composer