remove Docker Hub publish workflow

[tnsardesai]

Summary

ECR is the only registry we deploy from now. This deletes .github/workflows/docker.yml which built and pushed the image to Docker Hub on every main push — nothing consumes that image anymore.

Follow-up (not in this PR)

• delete repo secrets DOCKERHUB_USERNAME and DOCKERHUB_TOKEN
• scale the Railway service to 0, then delete it after a hold window

Test plan

• confirm build.yml (ECR) still runs and pushes on next main push
• confirm deploy.yml cross-repo dispatch still fires the AWS deploy

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Note

Low Risk
CI-only removal of an unused Docker Hub pipeline; no application or deploy logic changes in this diff.

Overview
Removes the Docker Hub CI workflow (.github/workflows/docker.yml) that previously built the nvidia-license-server-exporter image on PRs and pushed latest / SHA tags to Docker Hub on main pushes.

Deployment is now expected to rely solely on the existing ECR build path (build.yml) and AWS deploy dispatch (deploy.yml); this PR does not change those workflows.

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