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 
I assumed that the top-level Makefile also ran make in rocksdb, but I didn't verify it. Looks like we need another make in rocksdb after the git submodule update (or we figure out how to solve this correctly)