A basic docker recipie

Summary:
builds a docker image containing a ready to run cockroach executable

Lot's of things to improve in the future here:
- network port exposing
- automatic running of cockroach
- auto configuration of known docker nodes (maybe via etcd?)

Test Plan: used this dockerfile to build a docker image from scratch

Reviewers: Tobias

Reviewed By: Tobias

Subscribers: Tobias

Differential Revision: http://phabricator.cockroachdb.org/D77