Use Docker VOLUME for git repo

[cvrebert]

This is needed so that the repo and the node_modules are mutable and persistent across different runs of the Docker image.
Savage wasn't affected by this since it's a daemon and thus only uses one container.

[cvrebert]

Welp, that failed.
Looks like adding -v /host/path:/container/path to the docker run command in gruntworker.crontab is the easiest (albeit slightly annoying) proper way to do this.
This will unfortunately mean the git remote/config setup commands will have to be moved out of the Dockerfile.

[cvrebert]

Decided instead to just stop using Docker for this. Too much yak shaving.