Adjust Dockerfile style to be more consistent

This includes some of the orthogonal changes from https://github.com/docker/docker/pull/9481 which came from https://github.com/docker/docker/pull/9016 originally.

One of the biggest changes here is pinning blackfriday to a specific version on top of pinning go-md2man, and making sure we don't accidentally prefill our `GOPATH` with a freshly downloaded `go.net` which is `vendor`ed for Docker's own use too (since the freshly downloaded version would take precedence, and that could cause some unintended consequences).