Consistent dockerfile name for multiple HW support

[tarukumar]

Currently, we have Dockerfile names based on devices, except for CUDA. This seems to be because CUDA is considered a first-class citizen for vllm (and Torch), making it "the default" that everyone references. However, I believe it's better to maintain consistency in naming conventions for all device Dockerfiles, such as using Dockerfile.<device>. So i wanted to dicuss if it is good to do that for CUDA file to Dockerfile.cuda