dockerhub builds are always "dirty"

[stephen-soltesz]

The change from #90 works locally but appears to unconditionally fail during dockerhub builds. We first noticed this on 2020-06-30, though it's been this way on the platform since 2020-03-24.

Step 5/10 : RUN go get -v -ldflags "-X github.com/m-lab/go/prometheusx.GitShortCommit=$(git log -1 --format=%h)$(git diff --quiet || echo dirty)" github.com/m-lab/pusher
---> Running in 7ea47bd89a9b
�[91m fatal: not a git repository: /src/bpge2z8lqrw3la8xeglknwz/.git/modules/travis