images/alpine: Build for arm64

This commit switches the builder base image used for building the alpine
image to `gcr.io/k8s-testimages/gcb-docker-gcloud` in order to do
multi-arch (amd64 and arm64) builds via `docker buildx build`, yielding
a multi-arch manifest.

images/alpine/cloudbuild.yaml

@@ -1,19 +1,21 @@
 steps:
-  - name: gcr.io/cloud-builders/docker
+  - name: gcr.io/k8s-testimages/gcb-docker-gcloud
+    entrypoint: /buildx-entrypoint
     args:
     - build
-    - -t
-    - gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
+    - --tag=gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
+    - --platform=linux/amd64,linux/arm64/v8
     - --build-arg=IMAGE_ARG=gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
+    - --push
     - .
     dir: .
-  - name: gcr.io/cloud-builders/docker
+  - name: gcr.io/k8s-testimages/gcb-docker-gcloud
+    entrypoint: gcloud
     args:
-    - tag
+    - container
+    - images
+    - add-tag
     - gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
     - gcr.io/$PROJECT_ID/alpine:latest
 substitutions:
   _GIT_TAG: '12345'
-images:
-  - 'gcr.io/$PROJECT_ID/alpine:$_GIT_TAG'
-  - 'gcr.io/$PROJECT_ID/alpine:latest'