fix(php): install mbstring extension in php-fpm base images

composer install failed for php-fpm targets because stripe/stripe-php
requires ext-mbstring, which wasn't installed. apache-mod targets were
unaffected since they use the official PHP image, which bundles it.

Author: nccatoni

utils/build/docker/php/php-fpm/build.sh

@@ -31,7 +31,7 @@ nala update
 printf '#!/bin/sh\n\nexit 101\n' > /usr/sbin/policy-rc.d
 chmod +x /usr/sbin/policy-rc.d
 
-nala install -y --no-install-recommends tzdata publicsuffix curl apache2 libapache2-mod-fcgid jq ca-certificates git unzip php$PHP_VERSION-fpm php$PHP_VERSION-curl apache2 php$PHP_VERSION-mysql php$PHP_VERSION-pgsql php$PHP_VERSION-xml php$PHP_VERSION-mongodb
+nala install -y --no-install-recommends tzdata publicsuffix curl apache2 libapache2-mod-fcgid jq ca-certificates git unzip php$PHP_VERSION-fpm php$PHP_VERSION-curl apache2 php$PHP_VERSION-mysql php$PHP_VERSION-pgsql php$PHP_VERSION-xml php$PHP_VERSION-mongodb php$PHP_VERSION-mbstring
 
 rm -rf /usr/sbin/policy-rc.d
 rm -rf /var/lib/apt/lists/*