Update docker docs with ARM CUDA cross-compile

[mgoin]

I ran into this complication when trying to cross-compile ARM wheels on my x86 CUDA machine. After running this extra command, the following command works

DOCKER_BUILDKIT=1 docker build . \                       
  --file docker/Dockerfile \
  --target build \
  --platform "linux/arm64" \
  -t vllm/vllm-cuda-arm-build:latest \
  --build-arg max_jobs=66 \
  --build-arg nvcc_threads=2 \
  --build-arg torch_cuda_arch_list="9.0 10.0+PTX" \
  --build-arg vllm_fa_cmake_gpu_arches="90-real" --progress=plain

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[mgoin]

@simon-mo this is needed in order to do cross-compile for ARM using an x86 system

[houseroad]

Looks good to me.