Switch to Debian apt packages, pin nrfx to v2.9.0

- Replace ARM tarball download with gcc-arm-none-eabi from apt
- Drop TARGETARCH/GCC_VERSION build args — no longer needed
- Pin nrfx submodule to v2.9.0 which still bundles mdk/ headers
- nrfx 3+ dropped mdk/ from the repo

Dockerfile

@@ -1,23 +1,11 @@
 FROM debian:bookworm-slim
 
-ARG GCC_VERSION=13.2.rel1
-ARG TARGETARCH=amd64
-
 RUN apt-get update && apt-get install -y --no-install-recommends \
-        wget xz-utils cmake ninja-build \
+        gcc-arm-none-eabi \
+        binutils-arm-none-eabi \
+        libnewlib-arm-none-eabi \
+        cmake \
+        ninja-build \
     && rm -rf /var/lib/apt/lists/*
 
-RUN case "${TARGETARCH}" in \
-        amd64) _arch=x86_64  ;; \
-        arm64) _arch=aarch64 ;; \
-        *) echo "unsupported arch: ${TARGETARCH}" >&2 && exit 1 ;; \
-    esac \
-    && wget -qO /tmp/gcc.tar.xz \
-        "https://developer.arm.com/-/media/Files/downloads/gnu/${GCC_VERSION}/binrel/arm-gnu-toolchain-${GCC_VERSION}-${_arch}-arm-none-eabi.tar.xz" \
-    && tar -xf /tmp/gcc.tar.xz -C /opt \
-    && ln -s "/opt/arm-gnu-toolchain-${GCC_VERSION}-${_arch}-arm-none-eabi" /opt/arm-toolchain \
-    && rm /tmp/gcc.tar.xz
-
-ENV PATH="/opt/arm-toolchain/bin:${PATH}"
-
 WORKDIR /src