ci: add Dockerfile and README for CI image setup

CI/Dockerfile.arm64

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+FROM ubuntu:24.04
+
+ARG USER_ID
+ARG GROUP_ID
+ARG USER_NAME
+
+RUN if [ -n "$USER_ID" ] && [ -n "$GROUP_ID" ] && [ -n "$USER_NAME" ]; then \
+        groupadd -g "$GROUP_ID" "$USER_NAME" && \
+        useradd -m -u "$USER_ID" -g "$GROUP_ID" -d / "$USER_NAME" ; \
+    fi
+
+ENV ARCH=arm64
+ENV CROSS_COMPILE=aarch64-linux-gnu-
+
+COPY generate_boot_bins.sh /usr/bin/
+COPY build.sh /usr/bin/
+COPY make_fitimage.sh /usr/bin/
+
+RUN set -eux; \
+    apt-get update; \
+    apt-get install -y --no-install-recommends \
+        ca-certificates \
+        curl \
+        git \
+        binutils \
+        python3 \
+        locales \
+        kmod \
+        rsync \
+        systemd-ukify \
+        dosfstools \
+        mtools \
+        cpio \
+    ; \
+    rm -rf /var/lib/apt/lists/*; \
+    locale-gen en_US.UTF-8; \
+    update-locale LANG=en_US.UTF-8; \
+    \
+    curl "https://android.googlesource.com/platform/system/tools/mkbootimg/+/refs/heads/android12-release/mkbootimg.py?format=TEXT" \
+      | base64 --decode > /usr/bin/mkbootimg; \
+    chmod +x /usr/bin/mkbootimg; \
+    chmod 755 /usr/bin/generate_boot_bins.sh /usr/bin/build.sh /usr/bin/make_fitimage.sh; \
+    \
+    command -v ukify; \
+    command -v mkfs.vfat; \
+    command -v mcopy; \
+    command -v rsync
+
+# Fetch & unpack systemd-boot .deb
+ARG SYSTEMD_BOOT_DEB_URL="http://ports.ubuntu.com/pool/universe/s/systemd/systemd-boot-efi_255.4-1ubuntu8_arm64.deb"
+RUN set -eux; \
+    mkdir -p /artifacts/systemd; \
+    echo "Downloading systemd-boot from: ${SYSTEMD_BOOT_DEB_URL}"; \
+    curl -fsSL "${SYSTEMD_BOOT_DEB_URL}" -o /artifacts/systemd-boot-efi.deb; \
+    dpkg-deb -xv /artifacts/systemd-boot-efi.deb /artifacts/systemd; \
+    test -d /artifacts/systemd/usr/lib/systemd/boot/efi
+
+ENV LANG=en_US.UTF-8
+ENV LC_ALL=en_US.UTF-8

CI/README.md

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+# CI Docker Image
+
+This directory contains a Dockerfile used to build a reproducible CI image for generating ARM64 boot images.
+
+## Base Image
+- Ubuntu 24.04
+
+## Features
+- ARM64 cross-compilation support
+- Systemd ukify tooling
+- mkbootimg utility
+- FAT filesystem utilities
+- Required scripts pre-installed:
+  - generate_boot_bins.sh
+  - build.sh
+  - make_fitimage.sh
+
+## Build the Docker Image
+
+```bash
+docker build \
+  --build-arg USER_ID=$(id -u) \
+  --build-arg GROUP_ID=$(id -g) \
+  --build-arg USER_NAME=$(whoami) \
+  -t make-image-ci \
+  .