Add dedicated Dockerfile for Linux WebGPU EP plugin pipeline
The WebGPU EP has no CUDA dependency (Dawn uses Vulkan on Linux), so
having the plugin-linux-webgpu-stage.yml pipeline reuse the CUDA
inference Dockerfile pulled in TensorRT/cuDNN unnecessarily and was
missing libvulkan.so.1, causing the test job to fail with:
Couldn't load Vulkan: libvulkan.so.1: cannot open shared object file
Add a new Dockerfile under inference/x86_64/python/webgpu/ modeled on
the CPU Dockerfile, based on the CPU build-cache image, with an
additional 'dnf install vulkan-loader' step so Dawn can reach the
GPU's Vulkan ICD (injected by the NVIDIA Container Toolkit via
--gpus all at runtime).
Update all three jobs (build, package, test) in
plugin-linux-webgpu-stage.yml to use the new Dockerfile and switch
the docker_base_image default to the CPU base image.