add testability through docker

added a ./deploy/wrapper.sh which routes everything to the cockroach binary
except when called when with a single parameter "test", in which case we run
make test and make testrace.

Somewhat optimized the size of the resulting Docker image and set the entry
point to the new wrapper script, allowing
  `docker run "cockroachdb/cockroach" test
to run the tests inside of the container.
setup CircleCI to test through Docker with caching

The script will look for previously generated Docker images and use those in
the build of our Cockroach image. On the first run (or after the cache has been
cleared) we do a full build and store the layers.
In effect, this gets the total build time down to about two minutes, where half
of that is due to the need to run `docker load` on our currently 1.2GB sized
container.
refactor Dockerfile into a base image and deploy

The base image resides in cockroachdb/docker_base. This allows for CircleCI to
cache only this image locally, and squash it when creating the cache without
resorting to hacky shell scripting.

Updated circle.yml to do what is described above.