feat: add Grype scanner integration with dual-scanner deduplication

[Aj7Ay]

Closes #50

• Adds Grype (Anchore) as a second vulnerability scanner alongside Trivy via --scanner {trivy,grype,all}
• --scanner all runs both scanners and deduplicates results by CVE ID — CVEs confirmed by
  both scanners are marked as highest-priority findings
• Adds DOCKSEC_SCANNER environment variable for CI/CD default configuration (no code changes
  needed)
• Extends Docker Compose scanning to pass --scanner through to each service scan
• Adds Scanner Coverage section to HTML/JSON reports showing trivy-only, grype-only, and
  confirmed-by-both counts
• Adds colored scanner badges (Trivy / Grype / Both) to the HTML vulnerability table
• Adds Sources column to CSV reports
• Updates README with new scanner commands, feature descriptions, and comparison table row

Changes

File	Description
docksec/setup_external_tools.py	install_grype() for macOS/Linux/Windows
docksec/docker_scanner.py	scan_image_grype(), _parse_grype_output(),
_deduplicate_vulnerabilities(), scanner routing, scanner-scoped cache keys
docksec/cli.py	--scanner flag + DOCKSEC_SCANNER env var fallback
docksec/compose_scanner.py	Passes scanner param to each DockerSecurityScanner
docksec/report_generator.py	Scanner coverage stats, badge HTML, Sources column in CSV
docksec/templates/report_template.html	Scanner badge CSS +
{{SCANNER_COVERAGE_SECTION}} placeholder
tests/test_docker_scanner.py	21 new tests for Grype parsing, deduplication, scanner
routing
tests/test_report_generator.py	Updated CSV header assertions for new Sources column
README.md	New scanner section, feature descriptions, comparison table

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.53551% with 339 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@9bcbf4f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
docksec/docker_scanner.py	45.52%	207 Missing ⚠️
docksec/setup_external_tools.py	0.00%	63 Missing ⚠️
docksec/report_generator.py	65.51%	30 Missing ⚠️
docksec/cli.py	52.27%	21 Missing ⚠️
docksec/compose_scanner.py	77.50%	18 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #102   +/-   ##
=======================================
  Coverage        ?   63.73%           
=======================================
  Files           ?       19           
  Lines           ?     3441           
  Branches        ?        0           
=======================================
  Hits            ?     2193           
  Misses          ?     1248           
  Partials        ?        0           

Flag	Coverage Δ
unittests	63.73% <60.53%> (?)

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[advaitpatel]

Hello @Aj7Ay thank you for your contributions however your PR seems a bit outdated due to recent changes in the tool. Can you please rebase and update your PR accordingly? Thank you!