[Testing] Add integration tests using Docker containers

[chahe-dridi]

Create Docker-based integration tests that:
1. Spin up an Ubuntu 22.04 container
2. Run: bash scripts/linux/ubuntu.sh git
3. Assert that git --version succeeds inside the container

Use GitHub Actions matrix to test on ubuntu:22.04, kali:latest.

This would give us real end-to-end confidence that scripts actually work.

Files to create: 
- tests/integration/ubuntu.test.sh
- .github/workflows/integration.yml