Bind mounts fail when Docker is running on remote host

When Docker is running on a remote host, e.g. in an environment like [CircleCI](https://circleci.com/docs/2.0/building-docker-images/), the bind mount used to run the `backstopjs` docker container will fail:

> Error response from daemon: invalid mount config for type "bind": bind source path does not exist.

https://github.com/garris/BackstopJS/blob/61c18a6bed03b506c76aa0f23e116b7c3ae70a32/core/util/runDocker.js#L20

The recommended solution in such a case is typically to use volumes instead. Is there a reason backstopjs does not use volumes?