ci: add Docker credentials env vars and skip test if not set

[gaius-qi]

Description

This pull request improves the way Docker credentials are handled in both the CI workflow and the image preheat unit tests. The main changes ensure that Docker Hub credentials are securely passed to the test environment and that tests requiring authentication are skipped if credentials are missing.

CI/CD and Testing Improvements:

• The CI workflow (.github/workflows/ci.yml) now sets DOCKER_USERNAME and DOCKER_PASSWORD environment variables for the unit test step, allowing tests to authenticate with Docker Hub.

• The image preheat unit test (internal/job/image_test.go) now reads Docker credentials from environment variables and skips tests if they are not set, preventing authentication errors in environments without secrets configured.

• The ManifestRequest objects in the test cases are updated to include the Docker credentials, enabling authenticated requests to Docker Hub during testing. [1] [2]

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.12%. Comparing base (eda0bdc) to head (c51d991).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4687      +/-   ##
==========================================
- Coverage   28.66%   28.12%   -0.55%     
==========================================
  Files         232      232              
  Lines       23097    23097              
==========================================
- Hits         6620     6495     -125     
- Misses      16015    16161     +146     
+ Partials      462      441      -21     

Flag	Coverage Δ
unittests	28.12% <ø> (-0.55%)	⬇️

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see 1 file with indirect coverage changes

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[EvanCley]

lgtm