Update dockerfile directory

Author: sd109

.github/workflows/build-push-vllm-cpu.yml

@@ -41,6 +41,7 @@ jobs:
           IMAGE=ghcr.io/stackhpc/vllm-cpu:${{ inputs.vllm_ref }}
           docker build -f Dockerfile.cpu -t $IMAGE --shm-size=4g .
           docker push $IMAGE
+        working-directory: docker
 
   build_push_arm64_image:
     name: Build and push image