feat: Add Docker support

[robbeverhelst]

Description

This PR adds Docker support to enable easy self-hosting of GitHub Readme Streak Stats. The implementation provides a production-ready, secure Dockerfile with
minimal changes to the codebase, addressing community requests for containerized deployment options.

Fixes #622

Type of change

• New feature (added a non-breaking change which adds functionality)
• Updated documentation (updated the readme, templates, or other repo files)

How Has This Been Tested?

• Tested locally with a valid username
• Tested locally with an invalid username
• Successfully deployed to production Kubernetes cluster
• Verified all features work including PNG rendering with Inkscape
• Tested on both ARM64 and x86_64 architectures

Live demo available at: https://streak-stats.robbeverhelst.com/demo/

Checklist:

• I have checked to make sure no other pull requests
  are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Additional Notes

• Kept changes minimal (2 files) based on feedback from Added Docker Support and Heroku Nginx. #322
• Dockerfile includes security best practices:
  • Disabled directory indexing
  • Security headers (X-Content-Type-Options, X-Frame-Options)
  • Minimal attack surface with --no-install-recommends
  • Proper file permissions
• Full feature compatibility including SVG and PNG rendering
• Kubernetes manifests available for separate PR if desired

[ildyria]

@robbeverhelst Great job ! I also improved a bit on your docker image by pinning the versions. Tested locally to work.

[robbeverhelst]

Thx @ildyria! I resolved your comments!

[DenverCoder1]

Thanks for contributing!