Add a couple tweaks to the Dockerfile

- we don't need to add a "daemon" user to busybox; it already has one :)
- if we split out the "go get" from the "go install", we can have nice clean output on "docker build" of which dependencies we're pulling in and all the packages that get built :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #123 from tianon/dockerfile-tweaks

Add a couple tweaks to the Dockerfile