cmd/buildlet/stage0: kubernetes Makefile target

Change-Id: I77d5a42bbb0b467492e8527a11ed27dedfc56f6e
Reviewed-on: https://go-review.googlesource.com/15251
Reviewed-by: Evan Brown <evanbrown@google.com>

cmd/buildlet/stage0/Makefile

@@ -5,3 +5,11 @@ buildlet-stage0.windows-amd64: stage0.go
 buildlet-stage0.linux-arm-scaleway: stage0.go
 	GOOS=linux GOARCH=arm go build -o $@
 	cat $@ | (cd ../../upload && go run upload.go --public --cacheable=false go-builder-data/$@)
+
+# No staging vs prod targets here, because this file gets baked into
+# the Docker layers on gcr.io (which do have staging vs prod names).
+# That is, this file on GCS is only used at "docker build"-time, not
+# during day-to-day builds.
+buildlet-stage0.linux-amd64-kube: stage0.go
+	GOOS=linux GOARCH=amd64 go build -o $@
+	cat $@ | (cd ../../upload && go run upload.go --public --cacheable=false go-builder-data/$@)