Add Raspberry Pi (arm64) raspbian distro with its own bake target

[Copilot]

Adds a new raspbian distro targeting linux/arm64 only, using traditional Raspberry Pi firmware boot instead of GRUB EFI.

New files

• raspbian/Dockerfile — debian:bookworm-slim base; adds the RPi Foundation apt repo to pull raspberrypi-bootloader (GPU firmware: start4.elf, fixup4.dat, overlays, etc.); installs linux-image-arm64 + systemd networking

docker-bake.hcl

• New standalone target "rootfs-raspbian" and target "disk-image-raspbian" — linux/arm64 only, EFI_SIZE=256M (to fit firmware + kernel + initrd)
• Intentionally excluded from default group to avoid conflicts with the CI matrix *.platform override

Dockerfile.disk-image

• ARG DISTRO moved before the package install RUN so the GRUB condition works
• GRUB packages (grub-efi-*-bin, grub2-common) skipped when DISTRO=raspbian

build-image.sh

• BOOT_MNT variable replaces the hardcoded /boot/efi path — set to /boot/firmware for raspbian, /boot/efi otherwise; propagated through cleanup, mount, unmount, and fstab
• RPi boot path (triggered when DISTRO=raspbian): copies vmlinuz-* → kernel8.img and initrd.img-* → initrd.img onto the FAT32 partition, then writes:
  • config.txt — arm_64bit=1, kernel/initramfs pointers
  • cmdline.txt — PARTUUID-based root, serial console, rootwait

# Build the Raspberry Pi disk image
docker buildx bake disk-image-raspbian --allow=security.insecure
# → ./output/raspbian/disk.img

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