Add docs how to use options with docker

[MalteEbner]

I would add to the docs/README examples how to use the options together with the docker. I needed several minutes to try out and find out, this could have been easier. Furthermore, it required manually inspecting the container to find out that it contained a ./gpu_burn executable.

E.g. add this example to the README

docker run --rm --gpus all gpu_burn ./gpu_burn [OPTIONS] [TIME]