[ci] Use ArchLinux as base.

.gitlab-ci.yml

 # Select image from https://hub.docker.com/_/php/
-image: php:7.4
+image: archlinux:latest
 
 before_script:
-        - apt-get update && apt-get install -y zip unzip git
+        - pacman -Syu --noconfirm && pacman -S php7 curl
         - curl https://composer.github.io/installer.sig | tr -d '\n' > installer.sig
-        - php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
-        - php -r "if (hash_file('SHA384', 'composer-setup.php') === file_get_contents('installer.sig')) { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
-        - php composer-setup.php
-        - php -r "unlink('composer-setup.php'); unlink('installer.sig');"
-        - php composer.phar install --no-dev --optimize-autoloader
+        - php7 -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
+        - php7 -r "if (hash_file('SHA384', 'composer-setup.php') === file_get_contents('installer.sig')) { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
+        - php7 composer-setup.php
+        - php7 -r "unlink('composer-setup.php'); unlink('installer.sig');"
+        - php7 composer.phar install --no-dev --optimize-autoloader
 
 package:
   variables: