Switch to use nubs/phpunit base docker image.

tests/Dockerfile

@@ -1,16 +1,17 @@
-FROM base/archlinux
+FROM nubs/phpunit
+
+USER root
 
 RUN pacman --sync --refresh --noconfirm --noprogressbar --quiet
-RUN pacman --sync --noconfirm --noprogressbar --quiet php xdebug php-mongo git openssh
+RUN pacman --sync --noconfirm --noprogressbar --quiet php-mongo
 
-ADD provisioning/php/php-extensions.ini /etc/php/conf.d/extensions.ini
-ADD provisioning/php/xdebug.ini /etc/php/conf.d/xdebug.ini
-ADD provisioning/set-env.sh /set-env.sh
+# Get around a bug with docker registry where the owner is root for the home
+# user.  See https://github.com/docker/docker/issues/5892.
+RUN chown -R build /home/build
 
-RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+USER build
 
-VOLUME ["/code"]
-WORKDIR /code
+ADD provisioning/set-env.sh /home/build/set-env.sh
 
-ENTRYPOINT ["/set-env.sh"]
-CMD ["/code/build.php"]
+ENTRYPOINT ["/home/build/set-env.sh"]
+CMD ["./build.php"]