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 
Yes, but the "git submodule update" is occurring on line 66. If that changes the rocksdb code we won't rebuild rocksdb. Unless I'm missing something.