fix: keep docker stdout off JSON output

[clement-tourriere]

Summary

• Prevents docker scan stdout from leaking into JSON output, ensuring clean parseable JSON when using --json flag.

Test plan

• Unit tests updated in tests/unit/verticals/secret/test_scan_docker.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.05%. Comparing base (b0a6380) to head (5f6dc32).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1172   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         144      144           
  Lines        6254     6255    +1     
=======================================
+ Hits         5757     5758    +1     
  Misses        497      497           

Flag	Coverage Δ
unittests	92.05% <100.00%> (+<0.01%)	⬆️

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[agateau-gg]

Approved, but some changes seem unnecessary.