add volume_driver Docker configuration file option

## What does this MR do?
This adds support in `config.toml` for Docker's `volume-driver` option.

## Why was this MR needed?
Along with the existing support for Docker options like `volumes`, `devices`, etc., some setups need to access the `volume-driver` option (e.g. containers that wish to use NVIDIA GPUs using the [`nvidia-docker`plugin](https://github.com/NVIDIA/nvidia-docker/wiki/NVIDIA-driver).

## Are there points in the code the reviewer needs to double check?
None that I know of; it is quite straightforward, as it involves just reading one more string option from the configuration file and passing it directly to the `HostConfig` section to `go-dockerclient`.

## Does this MR meet the acceptance criteria?

- [x] Documentation created/updated
- Tests
  - [ ] Added for this feature/bug (I did not add any for this function, as I'm not sure where/how it might be appropriate; I'm willing to do so if someone can point me to where it might make sense to do so)
  - [ ] All builds are passing (will update once the builds run)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)

## What are the relevant issue numbers?
N/A

See merge request !365