Build post-training docker images in github workflow daily

[SurbhiJainUSC]

Description

Update UploadDockerImages.yml workflow to Build and push post-training docker images daily.

Tests

https://github.com/AI-Hypercomputer/maxtext/actions/runs/20474467869

Post-training stable image: http://shortn/_rt0cBqgxqR
Post-training nightly image: http://shortn/_wkJDacjip1
Tested by running GRPO: https://cloudlogging.app.goo.gl/RUyvtHr2H9U1jZNF9

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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