Docs: Add reproducible Unikernel examples and build documentation (Nginx, Redis, Httpreply)

[Nachiket-Roy]

Description

This PR introduces robust, reproducible build documentation and Dockerfiles for three popular unikernel applications (Nginx, Redis, and Httpreply). It significantly lowers the barrier to entry by providing users with exact steps and automated container configurations to build unikernel images via Unikraft and deploy them seamlessly over urunc.

Related issues

• Fixes Provide documentation and building steps for unikernel examples #107

How was this tested?

• Manually built all three examples from scratch using nerdctl build -t urunc--example . and docker build inside an Ubuntu WSL environment.
• Verified that the initramfs builds for Nginx and Redis correctly compile and embed the filesystem directly into the debug kernel binaries.
• Specifically tested the Redis conditional fallback mechanism to ensure it properly catches the compilation error, applies the sed patch, leverages the kraft object cache, and successfully re-links the final binary without hanging.

LLM usage

Gemini

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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