Add base archiso content, build script, Dockerfile, and docs updates

[Dzodah8772]

Motivation

• Provide a minimal, ready-to-use repository layout to build an Arch-based live ISO (ArchFlux) using archiso so the project can be built locally or in CI/containers.
• Include a reproducible Docker-based build path and a portable build script to simplify developer and CI workflows.
• Add sensible defaults and starter overlays so the repository is usable out of the box for further customization.

Description

• Add a minimal package list in packages.x86_64 with a small set of base live packages.
• Add starter airootfs overlays: airootfs/etc/hostname, airootfs/etc/motd, and airootfs/root/.bashrc to populate the live environment.
• Add a portable build helper scripts/build.sh that auto-installs archiso if missing, prepares an archiso profile from SRCTEMPLATE, applies packages.x86_64 and airootfs/ overlays when present, and runs mkarchiso to produce the ISO in out/; the script respects environment overrides (PROFILE_NAME, WORKDIR, OUTDIR, SRCTEMPLATE).
• Add a Dockerfile that installs archiso, copies the repository to /workspace, makes the build script executable, and invokes the build script to enable containerized builds.
• Add .gitignore entries to ignore generated work/, out/, and archflux/ build directories and temporary logs.
• Update README.md with quick-start instructions for local and Docker builds and document the packages.x86_64 and airootfs/ inputs.

Testing

• No automated tests were run for this change.

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Codex Task