use local path for postgres dir to work outside of linux hosts also.

[jonhadfield]

OSX with docker 1.12.0-a reports:

docker: Error response from daemon: Mounts denied:
The path /var/lib/buildbot_db
is not shared from OS X and is not known to Docker.
You can configure shared paths from Docker -> Preferences... -> File Sharing.
See https://docs.docker.com/docker-for-mac/osxfs/#namespaces for more info.

This PR moves the postgres dir mountpoint to the /buildbot_db as that should always be permitted.

[mention-bot]

@jonhadfield, thanks for your PR! By analyzing the annotation information on this pull request, we identified @tardyp to be a potential reviewer