[Feat] add model-downloader image and CI workflow for ghcr publishing

[samzong]

FILL IN THE PR DESCRIPTION HERE

FIX #678 (link existing issues this PR will resolve)

Support a dedicated multi-arch model-downloader image (Dockerfile + workflow wiring)

---

• Make sure the code changes pass the pre-commit checks.
• Sign-off your commit by using -s when doing git commit
• Try to classify PRs for easy understanding of the type of changes, such as [Bugfix], [Feat], and [CI].

Detailed Checklist (Click to Expand)

Thank you for your contribution to semantic-router! Before submitting the pull request, please ensure the PR meets the following criteria. This helps us maintain the code quality and improve the efficiency of the review process.

PR Title and Classification

Please try to classify PRs for easy understanding of the type of changes. The PR title is prefixed appropriately to indicate the type of change. Please use one of the following:

• [Bugfix] for bug fixes.
• [CI/Build] for build or continuous integration improvements.
• [Doc] for documentation fixes and improvements.
• [Feat] for new features in the cluster (e.g., autoscaling, disaggregated prefill, etc.).
• [Router] for changes to the vllm_router (e.g., routing algorithm, router observability, etc.).
• [Misc] for PRs that do not fit the above categories. Please use this sparingly.

Note: If the PR spans more than one category, please include all relevant prefixes.

Code Quality

The PR need to meet the following code quality standards:

• Pass all linter checks. Please use pre-commit to format your code. See README.md for installation.
• The code need to be well-documented to ensure future contributors can easily understand the code.
• Please include sufficient tests to ensure the change is stay correct and robust. This includes both unit tests and integration tests.

DCO and Signed-off-by

When contributing changes to this project, you must agree to the DCO. Commits must include a Signed-off-by: header which certifies agreement with the terms of the DCO.

Using -s with git commit will automatically add this header.

What to Expect for the Reviews

[netlify]

✅ Deploy Preview for vllm-semantic-router ready!

Name	Link
🔨 Latest commit	65601c5
🔍 Latest deploy log	https://app.netlify.com/projects/vllm-semantic-router/deploys/692dba5808ae0a000895e7cd
😎 Deploy Preview	https://deploy-preview-738--vllm-semantic-router.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 Root Directory

Owners: @rootfs, @Xunzhuo
Files changed:

• Dockerfile.model-downloader
• .github/workflows/docker-publish.yml

---

🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[samzong]

@rootfs @Xunzhuo

It seems that the reason CI fails is because there is no mirror (the first build has not been merged yet). Is there any solution suggestion?

[samzong]

I think it's can be split to 2 PRs.

Currently forcing build image and pushlish image.

Then upgrade helm chart.

[rootfs]

@samzong thanks! once you've have the container image, we'll add to the ghcr publish pipeline so you can use the published image in helm.

[samzong]

@rootfs Thanks. The image is actually built in this PR's workflow. Once this PR is merged, the model-downloader image will be available on ghcr.

After that I can send the follow-up PR to update the Helm chart.