Use DockerHub mirror when running system tests

See gh-31072

Author: wilkinsona

ci/tasks/run-system-tests.yml

@@ -13,11 +13,12 @@ params:
   GRADLE_ENTERPRISE_CACHE_USERNAME:
   GRADLE_ENTERPRISE_CACHE_PASSWORD:
   GRADLE_ENTERPRISE_URL: https://ge.spring.io
+  DOCKER_HUB_MIRROR:
 run:
   path: bash
   args:
     - -ec
     - |
       source /docker-lib.sh
-      start_docker
+      start_docker $DOCKER_HUB_MIRROR
       ${PWD}/git-repo/ci/scripts/run-system-tests.sh