Add documentation for keeping containers clean

[vertti]

Summary

• Add "Keeping Containers Clean" section to docs/usage.md with patterns for running preflight checks without adding bloat to final images
• Add brief note in README.md pointing to the new section

Covers three patterns:

• External validation: Run checks via docker run myapp preflight run
• Entrypoint script: Runtime checks before app starts
• Multi-stage build: Validate during build, ship clean final image

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (911bfc0) to head (040cfdc).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   88.80%   88.80%           
=======================================
  Files          31       31           
  Lines        1322     1322           
=======================================
  Hits         1174     1174           
  Misses        108      108           
  Partials       40       40           

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