Add Docker cross-compilation for webOS ARM (glibc 2.28)

Debian buster base matches the TV's glibc version. Includes fake
WEBOS_NDK wrapping standard cross-compiler, stub shared libraries
for luna-service2 and glib-2.0, and a build script that produces
a deployable ARM soft-float binary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/Dockerfile.build

@@ -0,0 +1,36 @@
+FROM debian:buster-slim
+
+# Buster is archived — point sources at archive.debian.org
+RUN sed -i 's|deb.debian.org|archive.debian.org|g; s|security.debian.org|archive.debian.org|g; /buster-updates/d' /etc/apt/sources.list && \
+    apt-get update && apt-get install -y --no-install-recommends \
+    curl ca-certificates pkg-config cmake git \
+    gcc libc6-dev gcc-arm-linux-gnueabi binutils-arm-linux-gnueabi libc6-dev-armel-cross \
+    python3 \
+    xz-utils && \
+    rm -rf /var/lib/apt/lists/*
+
+# Node.js 20 binary (supports glibc 2.28+, has import.meta.dirname)
+RUN curl -fsSL https://nodejs.org/dist/v20.18.1/node-v20.18.1-linux-x64.tar.xz | \
+    tar -xJ --strip-components=1 -C /usr/local && \
+    npm install -g pnpm@9
+
+# Rust with ARM soft-float target
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
+    sh -s -- -y --default-toolchain stable --profile minimal && \
+    . /root/.cargo/env && \
+    rustup target add armv7-unknown-linux-gnueabi
+
+# Create a fake WEBOS_NDK that wraps the standard cross-compiler
+RUN mkdir -p /opt/webos-ndk/bin && \
+    ln -s /usr/bin/arm-linux-gnueabi-gcc /opt/webos-ndk/bin/arm-webos-linux-gnueabi-gcc && \
+    ln -s /usr/bin/arm-linux-gnueabi-ar /opt/webos-ndk/bin/arm-webos-linux-gnueabi-ar && \
+    ln -s /usr/bin/arm-linux-gnueabi-strip /opt/webos-ndk/bin/arm-webos-linux-gnueabi-strip && \
+    mkdir -p /opt/webos-ndk/arm-webos-linux-gnueabi/sysroot/usr/include && \
+    cp -a /usr/arm-linux-gnueabi/include/* /opt/webos-ndk/arm-webos-linux-gnueabi/sysroot/usr/include/ && \
+    mkdir -p /opt/webos-ndk/arm-webos-linux-gnueabi/sysroot/usr/lib && \
+    cp -a /usr/arm-linux-gnueabi/lib/* /opt/webos-ndk/arm-webos-linux-gnueabi/sysroot/usr/lib/
+
+ENV WEBOS_NDK=/opt/webos-ndk
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+WORKDIR /build