drupal#73 Change docker compose project name to prefix drupal_...

drupal#73 Change docker compose project name to prefix drupal_ for deployed sites, so that they are different from pipeline project names

mixins.yml

@@ -27,6 +27,8 @@ stages:
 
 .preparecomposerplugins:
   before_script:
+    # Downgrade composer if required
+    - if [[ -n $DOWNGRADE_COMPOSER ]]; then composer self-update --$DOWNGRADE_COMPOSER; fi
     # Add the Drupal Vendor Hardening Composer Plugin.
     # see https://github.com/drupal/core-vendor-hardening
     - composer config --no-plugins --no-interaction allow-plugins.drupal/core-vendor-hardening true