Improve submodule versioning in Dockerfile.

Previously we would build against whatever revision was 'master' at the
time the docker image was built; now we use 'git submodule update'
to find the right version (while preserving as much as possible the
ability to build rocksdb only once)

@spencerkimball @cockroachdb/developers 
The main cockroach Makefile never builds rocksdb anymore. Am I misunderstanding how this is working?