cloud build: initialize support for running commands in Dockerfile

The Dockerfile needs to run apk, which fails during the cloud build
with:
  failed to solve: rpc error: code = Unknown desc = failed to load
  LLB: runtime execution on platform linux/ppc64le not supported

The fix is described in https://github.com/multiarch/qemu-user-static#getting-started

Author: pohly

.cloudbuild.sh

@@ -0,0 +1,11 @@
+#! /bin/bash
+
+# shellcheck disable=SC1091
+. release-tools/prow.sh
+
+if find . -name Dockerfile | grep -v ^./vendor | xargs --no-run-if-empty cat | grep -q ^RUN; then
+    # Needed for "RUN apk" on non linux/amd64 platforms.
+    (set -x; docker run --rm --privileged multiarch/qemu-user-static --reset -p yes)
+fi
+
+gcr_cloud_build