cmd/tipgodoc: make tipgodoc run on App Engine Managed VMs

At least in theory. We don't totally have it working yet. It does
run locally in the dev environment, though, which should be the same
as production, since it builds the Docker container locally.
But we're getting problems when pushing it to production.

Also some minor tweaks to the code with Andrew.

Change-Id: Id192669dbc8d3f86d9c8dad79764abd66e983895
Reviewed-on: https://go-review.googlesource.com/1761
Reviewed-by: Andrew Gerrand <adg@golang.org>