Use symfony 3.x for terminus

Author: tauno

Dockerfile

@@ -273,8 +273,10 @@ RUN export PATH="~/.composer/vendor/bin:$PATH" && \
     # Get Terminus
     && mkdir -p /var/www/.composer \
     && cd /var/www/.composer \
-    && curl -O https://raw.githubusercontent.com/pantheon-systems/terminus-installer/master/builds/installer.phar \
+    && curl -O https://raw.githubusercontent.com/pantheon-systems/terminus-instobo aller/master/builds/installer.phar \
     && php installer.phar install \
+    # Temporary fix until the next terminus release to use symfony 3.x
+    && composer require symfony/process:^3 \
 
     # Get Drupal console
     && curl https://drupalconsole.com/installer -L -o drupal.phar \