message: Upate dockerfiles/README.md for usages of `--gpus all`
  This PR updates dockerfiles/README.md for usages of `--gpus all`,
  as Docker version 19.03 changed.
diff: @@ -41,9 +41,13 @@ more info.
  # CPU-based images
  $ docker run -u $(id -u):$(id -g) -v $(pwd):/my-devel -it tf
  -# GPU-based images (set up nvidia-docker2 first)
  +# GPU-based images,
  +# 1) On Docker versions earlier than 19.03 (set up nvidia-docker2 first)
  $ docker run --runtime=nvidia -u $(id -u):$(id -g) -v $(pwd):/my-devel -it tf
  +# 2) On Docker versions including and after 19.03 (with nvidia-container-toolkit)
  +$ docker run --gpus all -u $(id -u):$(id -g) -v $(pwd):/my-devel -it tf
  +
  # Images with Jupyter run on port 8888 and need a volume for your notebooks
  # You can change $(PWD) to the full path to a directory if your notebooks
  # live outside the current directory.
  