cloud build: initial set of shared files

The approach taken here extends the existing support for
cross-compiling binaries on the build host and specifying the Go
compiler: Go is installed if needed (as in Prow testing), binaries are
build on the host, then one image is created for each platform, and
finally those are combined into a single multi-architecture image.

cloudbuild.sh

@@ -0,0 +1,6 @@
+#! /bin/bash
+
+# shellcheck disable=SC1091
+. release-tools/prow.sh
+
+gcr_cloud_build