cmd/gitmirror: cleanup and optimize dockerfile

Before:
REPOSITORY                            SIZE
gcr.io/go-dashboard-dev/gitmirror     1.32GB

After:
REPOSITORY                            SIZE
gcr.io/go-dashboard-dev/gitmirror     208MB

Updates golang/go#18817

Change-Id: I633fbbde2c5bba52535a8cfefdcb1f0532c45c06
Reviewed-on: https://go-review.googlesource.com/40571
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Kevin Burke <kev@inburke.com>
5 files changed