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.
Eventually maybe we'll want to have more than one image. I don't know if best practices around those needs have emerged, but should be fairly simple if we build things up on our base prereq layer.