[ci] add nightly docker build workflow

[zhulinJulia24]

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Motivation

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Modification

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[Copilot]

Pull request overview

Adds CI workflows to automate publishing nightly Docker images and syncing published images into an internal registry.

Changes:

• Added a new scheduled + manual docker-nightly workflow to build/push a nightly CUDA 12.8 image.
• Extended the existing Docker workflow to sync a published image into an internal registry.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
.github/workflows/docker.yml	Adds an internal-registry sync job after Docker image publication
.github/workflows/docker-nightly.yml	Introduces a scheduled/manual nightly Docker build + internal-registry sync workflow

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