Update README.md

The default IPEX and PyTorch versions are v1.9.0

Author: EikanWang

docker/README.md

@@ -10,6 +10,6 @@
 
   ```console
   $ cd $DOCKERFILE_DIR
-  $ DOCKER_BUILDKIT=1 docker build --build-arg IPEX_VERSION=v1.8.0 --build-arg PYTORCH_VERSION=v1.8.0 -t intel-extension-for-pytorch:test .
+  $ DOCKER_BUILDKIT=1 docker build -t intel-extension-for-pytorch:test .
   $ docker run intel-extension-for-pytorch:test python -c "import torch;import intel_pytorch_extension as ipex;print('torch:', torch.__version__,' ipex:',ipex.__version__)"
   ```