Move db storage out of docker containers

[kevsul]

Overview

Previously when running docker-compose, the rocksdb data dir was stored inside the childchain, watcher and watcher_info docker containers, which meant that they were lost every time the container was restarted, so the services were out of sync with local geth, which meant that any time you had to start a new chain and redeploy the contracts again on every restart.

Moving the rocksdb data dir out of the container makes it persistent, so that the services can be restarted and they will pick up where they left off.

Changes

• In docker-compose.yaml, mount /data as a volume and use it as the rocksdb data dir for childchain, watcher and watcher_info. Same pattern as the geth container.

[unnawut]

Could you also update Makefile's docker-nuke?

- docker-compose down --remove-orphans
+ docker-compose down --remove-orphans --volumes

So the volume doesn't hang around when geth snapshot is wiped.

[coveralls]

Coverage remained the same at 77.581% when pulling b127860 on kevin/move_rocksdb_out_of_docker_containers into 4389bce on master.