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>
6 files changed