images/alpine: Build for arm64

This commit adds a Google Cloud Build definition for building alpine
arm64 images and pushing them as a manifest list together with their
amd64 counterparts.

images/alpine/cloudbuild.yaml

@@ -9,11 +9,31 @@ steps:
     dir: .
   - name: gcr.io/cloud-builders/docker
     args:
-    - tag
+    - build
+    - -t
+    - gcr.io/$PROJECT_ID/alpine:$_GIT_TAG-arm64
+    - --build-arg=ARCH=arm64
+    - --build-arg=IMAGE_ARG=gcr.io/$PROJECT_ID/alpine:$_GIT_TAG-arm64
+    - .
+  - name: 'gcr.io/cloud-builders/docker'
+    env: ['DOCKER_CLI_EXPERIMENTAL=enabled']
+    args: 
+    - manifest
+    - create
+    - gcr.io/$PROJECT_ID/alpine:latest
+    - --amend
     - gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
+    - --amend
+    - gcr.io/$PROJECT_ID/alpine:$_GIT_TAG-arm64
+  - name: 'gcr.io/cloud-builders/docker'
+    env: ['DOCKER_CLI_EXPERIMENTAL=enabled']
+    args: 
+    - manifest
+    - push
     - gcr.io/$PROJECT_ID/alpine:latest
 substitutions:
   _GIT_TAG: '12345'
 images:
   - 'gcr.io/$PROJECT_ID/alpine:$_GIT_TAG'
+  - 'gcr.io/$PROJECT_ID/alpine:$_GIT_TAG-arm64'
   - 'gcr.io/$PROJECT_ID/alpine:latest'