cmd/pubsubhelper: change Dockerfile to FROM scratch

Before:
REPOSITORY                             SIZE
gcr.io/go-dashboard-dev/pubsubhelper   834MB

After:
REPOSITORY                             SIZE
gcr.io/go-dashboard-dev/pubsubhelper   12.3MB

Updates golang/go#18817

Change-Id: Ibf1f33a04043c0defddd7fe6b8d0455092044ac0
Reviewed-on: https://go-review.googlesource.com/40574
Reviewed-by: Kevin Burke <kev@inburke.com>
diff --git a/cmd/pubsubhelper/README b/cmd/pubsubhelper/README
new file mode 100644
index 0000000..d20742b
--- /dev/null
+++ b/cmd/pubsubhelper/README
@@ -0,0 +1,7 @@
+## Running with Docker locally
+
+docker run --rm -it \
+    -p 80:80 \
+    -p 25:25 \
+    -p 443:443 \
+    gcr.io/go-dashboard-dev/pubsubhelper:latest [any additional pubsubhelper flags]