all: fix vet errors

Replace oauth2.NoContext (deprecated) with context.Background(),
which has been available for two consecutive releases.

Add more cloud.google.com/go packages to the cmd/coordinator
Dockerfile to fix an error building cmd/coordinator. A dependency is
not present in master of cloud.google.com/go, but was present in the
older revision, and was not getting checked out correctly during the
"go get" step. In addition, we were failing to fetch dependencies for
some packages that coordinator depends on. I added instructions for
hopefully doing this more systematically in the future.

Fix the gitmirror Dockerfile which has the same problem.

Change-Id: Id6c2220482350a686b87742ec7915c457a689e52
Reviewed-on: https://go-review.googlesource.com/40852
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
10 files changed