support docker as a "runtime"

Instead of running unfurl in a python virtual machine we should have an option to have it run inside of docker. The runtime specifier could be "docker:image" (instead "venv:"), "docker:" alone would use a default container image we supply.

Here's some pseudocode for invoking a container with docker so that it has access to the local file system and the local docker daemon:

root_dir = project_root
reldir = os.path.relpath('.', root_dir)
exec docker run -it --rm \
    -u $(id -u):$(id -g) \
    -e HOME="/home/$USER" -e USER=$USER -v "$HOME":"/home/$USER" \
   -v "$root_dir":/data \
    -w /data/$reldir \ # workdir is cwd in container
    -v /var/run/docker.sock:/var/run/docker.sock
    $CONTAINER_NAME $unfurl_cmd

Of course the container would have to be trusted when used in this manner. The entry point for adding this support is [_remoteCmd in __main__](https://github.com/onecommons/unfurl/blob/master/unfurl/__main__.py#L217