docker: Install composer 2.x

We have switched to composer 2 everywhere and while 1.x should still work
we don't test with it. So install composer 2 in the docker image.

docker-dev/php-fpm/Dockerfile

@@ -41,6 +41,13 @@ STOPSIGNAL SIGQUIT
 ARG UID
 RUN useradd -u "$UID" -ms /bin/bash user
 RUN echo 'user ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
+
+# Install a newer composer
+RUN mkdir -p /home/user/.local/bin
+RUN wget https://raw.githubusercontent.com/composer/getcomposer.org/bfd95e2a4383ee9cf7c058c2df29d7acb5f86d77/web/installer  -O - -q | php --
+RUN mv composer.phar /home/user/.local/bin/composer
+ENV PATH "/home/user/.local/bin:${PATH}"
+
 USER user
 
 RUN git config --global url."https://gitlab.tugraz.at/".insteadOf "git@gitlab.tugraz.at:"