Update Dockerfile to Multi-Stage

[multi-stager]

Proposed change

Hi,

Thanks for maintaining OWASP Nettacker. I represent a research group investigating multi-stage builds. We recently refactored your Dockerfile to a multi-stage Dockerfile and found that it brings the following benefits:

✅ Reduced final image size (from 642MB to 137MB)
✅ Minimized image vulnerabilities (verified via Trivy, from 10 to 1)

We hope this small improvement to the Dockerfile will be useful to you :)

Thanks,

Type of change

• New core framework functionality
• Bugfix (non-breaking change which fixes an issue)
• Code refactoring without any functionality changes
• New or existing module/payload change
• Localization improvement
• Dependency upgrade
• Documentation improvement

Checklist

• I've followed the [contributing guidelines][contributing-guidelines]
• I've run make pre-commit, it didn't generate any changes
• I've run make test, all tests passed locally