Provides a reproducible Debian-based build environment with:
- Clang + LLD for cross-compilation to i386-elf targets
- GRUB 2 tools (grub-pc-bin, xorriso, mtools) for bootable
ISO and floppy image generation
- QEMU (i386) for integration test execution
- CMake + Ninja for the build system
The Dockerfile includes a self-test that verifies Clang + lld can
produce a valid ELF 32-bit i386 binary before the image is accepted.
