Shared build process for bootstrap.sh and Docker...

as well as some shuffling things around. After this merges, we will have automated builds of a base image (all the dependencies), a deployable image (still fairly large unfortunately) and Spencer's dnsmasq image, which I have moved into cockroachdb/docker-builds.
I'm a little confused as to why godeps.sh and vendor.sh are located in the seutp/ directory here but in build/ in the PR?