[Docs] Add README to the build docker image (vllm-project#8825)

Dockerfile

@@ -75,6 +75,7 @@ COPY csrc csrc
 COPY setup.py setup.py
 COPY cmake cmake
 COPY CMakeLists.txt CMakeLists.txt
+COPY README.md README.md
 COPY requirements-common.txt requirements-common.txt
 COPY requirements-cuda.txt requirements-cuda.txt
 COPY pyproject.toml pyproject.toml