feat(report): add markdown report format for CI/CD

[ALDRIN121]

Description

Adds Markdown as a supported DockSec report format, allowing scan results to be published directly into pull request comments, CI/CD job summaries, and other Markdown-compatible interfaces — no more jq-based conversion pipelines.

docksec path/to/Dockerfile --format markdown

The report reuses the same normalized scan data as the existing HTML/PDF/JSON/CSV exporters:

• Severity Summary: vulnerability counts grouped by severity (zero-count severities omitted)
• Vulnerabilities: readable table with ID, Severity, Package, Installed Version, Fixed Version, and Title (pipe characters escaped so rows stay intact)

Markdown is opt-in: the default report bundle stays json, csv, pdf, html so existing workflows are untouched, and it can be combined with other formats (--format json,markdown).

Closes #159

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code style update
• Code refactoring
• Performance improvement
• Test update
• Build / CI configuration
• Security fix

How Has This Been Tested?

• Unit tests
• Manual testing

Test Configuration:

• Python version: 3.12.7
• Operating System: macOS (Darwin 25.6.0)
• DockSec version: main @ 77c0cd1

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project (Black line-length 88, conventional commits).
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation (README, CHANGELOG, CLI help, config schema, action.yml).
• My changes generate no new warnings or errors (ruff check passes).
• I have added tests that prove my fix is effective or that my feature works (7 new tests).
• All existing tests pass (256 passed, 2 skipped).
• Any dependent changes have been merged and published in downstream modules.

Screenshots (if applicable)

N/A

Related Issues / PRs

Relates to #159

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[ALDRIN121]

Hi @advaitpatel — this PR implements #159 (Markdown report format for CI/CD). Would appreciate a review when you get a chance. All checks and the full test suite pass locally (256 passed, 2 skipped).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.37288% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.19%. Comparing base (a2fe184) to head (1f17879).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
docksec/report_generator.py	82.69%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   78.95%   80.19%   +1.24%     
==========================================
  Files          26       32       +6     
  Lines        4158     5101     +943     
==========================================
+ Hits         3283     4091     +808     
- Misses        875     1010     +135     

Flag	Coverage Δ
unittests	80.19% <92.37%> (+1.24%)	⬆️

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[advaitpatel]

lgtm