cmd/coordinator: change Dockerfile to FROM scratch

Before:
REPOSITORY                            SIZE
gcr.io/go-dashboard-dev/coordinator   895MB

After:
REPOSITORY                            SIZE
gcr.io/go-dashboard-dev/coordinator   20.5MB

Updates golang/go#18817

Change-Id: I144923fcd85aab5f1aaaa3f012143bac45f5086b
Reviewed-on: https://go-review.googlesource.com/40570
Reviewed-by: Kevin Burke <kev@inburke.com>
4 files changed