[CI] Add auto update workflow for Dockerfile graph

[WineChord]

This PR adds a GitHub Action workflow that automatically maintains the Dockerfile dependency graph visualization:

• Adds .github/workflows/update-dockerfile-graph.yml which:
  • Monitors changes to Dockerfile
  • Automatically regenerates the dependency graph
  • Creates PR when graph needs updating
  • Ensures documentation stays in sync with Dockerfile changes

The workflow helps maintain accurate documentation by automatically updating the
visualization in docs/source/assets/contributing/ whenever the Dockerfile changes.

This automation reduces maintenance overhead and prevents the documentation from
becoming outdated.

Fix #11880

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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 do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[WineChord]

I carefully designed the title and commit message to resemble the existing ones.

[hmellor]

Instead of a GitHub Action, could this be written as a pre-commit hook (which is kept in tools/) which is called only when the file docker/Dockerfile is changed?