CircleCI Docker testing with caching

Moved parts of the Dockerfile to https://github.com/cockroachdb/docker_base. This allows for a clean CircleCI build that caches the base image (=our prereqs for "make build"), bringing the total build time to around a minute and the tests to maybe another minute or two (our race tests are getting slower these days, especially kv).

We may want to move the base Dockerfile around a bit but since I haven't been added to our Docker organization (https://registry.hub.docker.com/u/cockroachdb/cockroach/, who runs this and can add me by the way?) yet, I kept it simple for now and didn't register it yet (which also means that from a fresh repo, you can't just "docker build ."). We'll want to have automated builds from the base and deploy images after a push.
In the process of changing this we should also migrate this into the organization:
https://registry.hub.docker.com/u/spencerkimball/dnsmasq/

Feel free to play around with this, this is the result of a 10hr session late last night and probably not perfect.
For actual deployments in the future, we will likely want to concoct something very small (right now running at over 800MB - insane for an actual deploy) that doesn't contain all the development stuff. Should be doable - build a static rocksdb library, build the cockroach binary, add both to a very minimal Docker image and  we should end up with something ridiculously small.
See https://medium.com/@kelseyhightower/optimizing-docker-images-for-static-binaries-b5696e26eb07

We probably want to build the static libs that go into the minimal container in our development Container. Maybe Dind will be useful: https://github.com/jpetazzo/dind