feat: install PHP intl extension

The extension is required by the SDK generator. Without it, the SDK generator
would throw an error like:

    Class Normalizer not found

Dockerfile

@@ -191,7 +191,7 @@ RUN \
   docker-cli \
   libgomp \
   git \
-  && docker-php-ext-install sockets pdo_mysql pdo_pgsql \
+  && docker-php-ext-install sockets pdo_mysql pdo_pgsql intl \
   && apk del .deps \
   && rm -rf /var/cache/apk/*