[hamlet] Update to php 8.1 (TechEmpower#6915)

Author: joanhey

frameworks/PHP/hamlet/benchmark_config.json

@@ -15,7 +15,7 @@
       "database": "mysql",
       "framework": "hamlet",
       "language": "PHP",
-      "flavor": "PHP8",
+      "flavor": "PHP8.1",
       "orm": "micro",
       "platform": "FPM/FastCGI",
       "webserver": "nginx",

frameworks/PHP/hamlet/hamlet.dockerfile

@@ -1,13 +1,13 @@
 FROM ubuntu:20.04
 
-ENV PHP_VERSION 8.0
+ENV PHP_VERSION 8.1
 ARG DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update -yqq && apt-get install -yqq software-properties-common > /dev/null
 RUN LC_ALL=C.UTF-8 add-apt-repository ppa:ondrej/php
 RUN apt-get update -yqq > /dev/null && \
     apt-get install -yqq nginx git unzip curl \
-    php${PHP_VERSION}-cli php${PHP_VERSION}-fpm php${PHP_VERSION}-apcu php${PHP_VERSION}-pdo-mysql > /dev/null
+    php${PHP_VERSION}-cli php${PHP_VERSION}-fpm php${PHP_VERSION}-apcu php${PHP_VERSION}-pdo-mysql php${PHP_VERSION}-dev > /dev/null
 
 COPY --from=composer:latest /usr/bin/composer /usr/local/bin/composer