Revert cloudbuild to gcr.io/k8s-testimages/gcb-docker-gcloud (#2155)

We can't use the vanilla docker image because we also need the `gcloud`
binary.

Author: mdbooth

cloudbuild.yaml

@@ -9,20 +9,25 @@ options:
   substitution_option: ALLOW_LOOSE
   machineType: 'N1_HIGHCPU_8'
 steps:
-  - name: gcr.io/cloud-builders/docker
+  - name: gcr.io/k8s-testimages/gcb-docker-gcloud
     entrypoint: bash
+    env:
+    # default cloudbuild has HOME=/builder/home and docker buildx is in /root/.docker/cli-plugins/docker-buildx
+    # set the home to /root explicitly to if using docker buildx
+    - HOME=/root
     args:
     - -c
     - |
       set -xeuo pipefail
 
-      # Initialise a new builder instance
-      docker buildx create --use
-
       # Create docker credentials for pushing to gcr.io from our inherited
       # gcloud credentials
       gcloud auth configure-docker
 
+      # Run the image's buildx entrypoint to initialise the build environment
+      # appropriately for the image before running make
+      /buildx-entrypoint version
+
       make push-multiarch-images \
         REGISTRY=gcr.io/$PROJECT_ID \
         VERSION=$_SHORT_TAG