Allow DOCKER_VOLUMES environment variable to contain multiple values

It would be very useful if you could pass multiple values using the DOCKER_VOLUMES environment variable similar how it works when you pass --docker-volumes multiple time. Currently, it's not possible to pass multiple values. Calling strings.Split(DockerConfig.Volumes, ",") would probably be enough, then the environment variable could contain multiple values separated by comma. This might be useful for other variables too.