run_tests.py: support downloading base image from dockerhub

tools/run_tests/dockerize/build_docker_and_run_tests.sh

@@ -47,12 +47,19 @@ mkdir -p /tmp/xdg-cache-home
 # Inputs
 # DOCKERFILE_DIR - Directory in which Dockerfile file is located.
 # DOCKER_RUN_SCRIPT - Script to run under docker (relative to grpc repo root)
+# DOCKERHUB_ORGANIZATION - If set, pull a prebuilt image from given dockerhub org.
 
 # Use image name based on Dockerfile location checksum
 DOCKER_IMAGE_NAME=$(basename $DOCKERFILE_DIR)_$(sha1sum $DOCKERFILE_DIR/Dockerfile | cut -f1 -d\ )
 
+if [ "$DOCKERHUB_ORGANIZATION" != "" ]
+then
+  DOCKER_IMAGE_NAME=$DOCKERHUB_ORGANIZATION/$DOCKER_IMAGE_NAME
+  docker pull $DOCKER_IMAGE_NAME
+else
   # Make sure docker image has been built. Should be instantaneous if so.
   docker build -t $DOCKER_IMAGE_NAME $DOCKERFILE_DIR
+fi
 
 # Choose random name for docker container
 CONTAINER_NAME="run_tests_$(uuidgen)"