[CI/Build] refactor dockerfile & fix pip cache

[CI/Build] fix pip cache with vllm_nccl & refactor dockerfile to build wheels (#3859)

.buildkite/test-pipeline.yaml

@@ -90,7 +90,7 @@ steps:
   - bash run-benchmarks.sh
 
 - label: Documentation Build
-  working_dir: "/vllm-workspace/docs"
+  working_dir: "/vllm-workspace/test_docs/docs"
   no_gpu: True
   commands:
   - pip install -r requirements-docs.txt