Add Ubuntu Server autoinstall support with local ISO boot

[synacktraa]

Key Changes

• Removed multi-OS support: Eliminated define.sh, focusing solely on Ubuntu
• Added cloud-init configuration: ubuntu-desktop-minimal, automated partitioning, default user: docker
• ISO remastering with pycdlib: Patches GRUB configuration in-place by adding autoinstall kernel parameters

Custom GRUB Configuration

• Added assets/grub.cfg with single autoinstall boot entry
• Refactored remaster_iso.py to support custom grub.cfg replacement

Enhanced Autoinstall User-Data

• Switch storage layout from LVM to direct for simpler partitioning
• Install ubuntu-desktop-minimal via late-commands for better control
• Enable boot splash screen and configure GDM auto-login
• Remove unused server packages (ubuntu-server, byobu, vim, openssh-server, etc.)
• Disable LTS upgrade prompts and setup unattended-upgrades
• Disable sleep, suspend, hibernate via systemd
• Configure GNOME power settings via dconf to prevent idle timeout, screensaver, and screen lock

Golden Image Support

• Check for ubuntu.boot marker to skip reinstallation when golden image exists
• Save remastered ISO to storage as ubuntu.<size>.iso for reuse
• Create ubuntu.base file with ISO filename for tracking
• Use dynamic $BOOT path instead of hardcoded /custom.iso

Graceful Shutdown

• Added power.sh with ACPI signal handling for graceful VM shutdown
• Updated entry.sh to run QEMU in background with terminal monitoring
• Trap handlers for SIGTERM, SIGHUP, SIGINT, SIGABRT, SIGQUIT

Build Fix

• Run dos2unix on scripts and assets in Dockerfile to fix CRLF line endings

Testing

Tested with Ubuntu 22.04.05 LTS Server ISO - fully automated installation with:

• No user interaction required
• Auto-login to desktop
• No sleep/suspend/idle timeout after boot
• Graceful shutdown on container exit
• Golden image reboot support