feat(docker): add ARM64 cross-compilation support via Docker

Add Dockerfile and build script for Linux aarch64 cross-compilation targeting ARM64 devices like Raspberry Pi. Includes documentation updates explaining the build process and troubleshooting tips.

The changes enable building Palladium binaries for ARM64 architecture using cross-compilation in a Docker container, with output binaries placed in the build directory.

docker-build/Dockerfile.linux-aarch64

@@ -0,0 +1,16 @@
+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND=noninteractive TZ=UTC
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential libtool autotools-dev automake autoconf pkg-config \
+    bsdmainutils python3 curl ca-certificates git unzip zip file rsync \
+    make cmake patch \
+    g++-aarch64-linux-gnu binutils-aarch64-linux-gnu \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /src
+
+COPY . /src
+
+CMD ["/bin/bash"]

docker-build/README.md

@@ -64,6 +64,77 @@ docker run --rm -it -v "$(pwd)/../build/linux-x86_64":/out palladium-builder:lin
 
 **Missing dependencies:** The Dockerfile installs all required build dependencies automatically
 
+## Linux aarch64 Build
+
+### Overview
+
+This build creates Linux aarch64 (ARM64) binaries specifically designed for single board computers like the popular Raspberry Pi and other ARM-based devices. The build process uses cross-compilation in a Docker container:
+
+1. **Builds a Docker image** (`palladium-builder:linux-aarch64-ubuntu20.04`) containing ARM64 cross-compilation toolchain
+2. **Copies the entire repository** into the container during image build
+3. **Cross-compiles for ARM64** using the `depends` system with `aarch64-linux-gnu` target
+4. **Outputs binaries** to a mounted volume for host access
+
+### Quick Start
+
+```bash
+cd docker-build
+chmod +x build-linux-aarch64.sh
+./build-linux-aarch64.sh
+```
+
+ARM64 binaries will be available in `../build/linux-aarch64/` directory.
+
+### Prerequisites
+
+- Docker installed and running ([installation guide](https://docs.docker.com/get-docker/))
+- Sufficient disk space for the build process (at least 12 GB free)
+- Internet connection for downloading dependencies and ARM64 cross-compilation toolchain
+
+### Target Devices
+
+This build is optimized for:
+- **Raspberry Pi 4** and newer models
+- **Raspberry Pi 3** (64-bit mode)
+- Other ARM64-based single board computers
+- ARM64 Linux servers and workstations
+
+### Produced Binaries
+
+The following ARM64 binaries are built and copied to the output directory:
+
+- `palladiumd`: Main daemon
+- `palladium-cli`: Command-line client
+- `palladium-tx`: Transaction utility
+- `palladium-wallet`: Wallet utility
+- `palladium-qt`: GUI application (if Qt dependencies are available)
+
+### Output Directory
+
+Binaries are placed in: `../build/linux-aarch64/` (relative to the docker-build directory)
+
+### Cross-Compilation Details
+
+- **Target Triple**: `aarch64-linux-gnu`
+- **Toolchain**: GCC ARM64 cross-compiler
+- **Dependencies**: Built using the `depends` system for ARM64 target
+- **Configuration**: Uses `CONFIG_SITE` for proper cross-compilation setup
+
+### Troubleshooting
+
+**Permission errors:** The build script automatically handles file permissions using host UID/GID
+
+**Build failed:** Run interactive container for debugging:
+```bash
+docker run --rm -it -v "$(pwd)/../build/linux-aarch64":/out palladium-builder:linux-aarch64-ubuntu20.04 bash
+```
+
+**Cross-compilation issues:** Ensure the ARM64 cross-compilation toolchain is properly installed in the container
+
+**Missing binaries:** Check that the configure script detected the ARM64 target correctly
+
+**Docker build issues:** Ensure Docker has sufficient resources and the daemon is running
+
 ## Windows x86_64 Build
 
 ### Overview

docker-build/build-linux-aarch64.sh

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+IMAGE_NAME="palladium-builder:linux-aarch64-ubuntu20.04"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_DIR="$(cd "${SCRIPT_DIR}/.." && pwd)"
+OUT_DIR="${REPO_DIR}/build/aarch64"
+HOST_TRIPLE="aarch64-linux-gnu"
+
+HOST_UID="$(id -u)"
+HOST_GID="$(id -g)"
+
+echo "[*] Build image including ALL repository files (COPY . /src)..."
+
+docker build --platform=linux/amd64 \
+  -t "$IMAGE_NAME" \
+  -f "${SCRIPT_DIR}/Dockerfile.linux-aarch64" \
+  "${REPO_DIR}"
+
+mkdir -p "${OUT_DIR}"
+
+echo "[*] Start container: build COMPLETED in container; mount ONLY output..."
+docker run --rm --platform=linux/amd64 \
+  -e HOST_UID="${HOST_UID}" -e HOST_GID="${HOST_GID}" \
+  -v "${OUT_DIR}":/out \
+  "$IMAGE_NAME" \
+  bash -c "
+    set -euo pipefail
+    cd /src
+
+    echo '[*] depends (HOST=${HOST_TRIPLE})...'
+    cd depends && make HOST=${HOST_TRIPLE} -j\$(nproc) && cd ..
+
+    echo '[*] autogen/configure...'
+    [[ -x ./autogen.sh ]] && ./autogen.sh
+    [[ -f ./configure ]] || { echo 'configure not found: autogen failed'; exit 1; }
+
+    ./configure --prefix=\$PWD/depends/${HOST_TRIPLE} \
+                --host=${HOST_TRIPLE} \
+                --enable-glibc-back-compat \
+                --enable-reduce-exports \
+                CC=${HOST_TRIPLE}-gcc CXX=${HOST_TRIPLE}-g++ \
+                AR=${HOST_TRIPLE}-ar RANLIB=${HOST_TRIPLE}-ranlib STRIP=${HOST_TRIPLE}-strip \
+                LDFLAGS='-static-libstdc++'
+
+    echo '[*] make...'
+    make -j\$(nproc)
+
+    echo '[*] copy binaries to /out...'
+    mkdir -p /out
+    for b in src/palladiumd src/palladium-cli src/palladium-tx src/palladium-wallet src/qt/palladium-qt; do
+      [[ -f \"\$b\" ]] && install -m 0755 \"\$b\" /out/
+    done
+
+    # Add permissions to host user
+    chown -R \${HOST_UID:-0}:\${HOST_GID:-0} /out
+
+    echo '[*] build COMPLETED (all binaries in container) → /out'
+  "
+
+echo "[*] Binaries aarch64 available in: ${OUT_DIR}"