Fix Dockerfile: skip cross-compilers when building natively

gcc-aarch64-linux-gnu is an x86→arm64 cross-compiler; it doesn't exist
on arm64 hosts (like the Pi). Only install cross-toolchains and set cargo
linker config when BUILDPLATFORM != TARGETPLATFORM.

infra/Dockerfile.server

@@ -1,18 +1,23 @@
 # syntax=docker/dockerfile:1
 # ── Build stage ───────────────────────────────────────────────────────────────
-# Always run the compiler on the *build* host for speed; cross-compile to target.
+# Run the compiler on the *build* host; cross-compile to target when needed.
 FROM --platform=$BUILDPLATFORM rust:1.86-slim-bookworm AS builder
 
+ARG BUILDPLATFORM
+ARG TARGETPLATFORM
 ARG TARGETARCH
 ARG TARGETVARIANT
 
-# Cross-compilation toolchains for every supported target.
+# Install cross-compilation toolchains only when actually cross-compiling.
-RUN apt-get update && apt-get install -y \
+RUN apt-get update && apt-get install -y pkg-config && \
-    pkg-config \
+    if [ "${BUILDPLATFORM}" != "${TARGETPLATFORM}" ]; then \
-    gcc-aarch64-linux-gnu \
+      case "${TARGETARCH}:${TARGETVARIANT}" in \
-    gcc-arm-linux-gnueabihf \
+        "arm64:")  apt-get install -y gcc-aarch64-linux-gnu    ;; \
-    gcc-arm-linux-gnueabi \
+        "arm:v7")  apt-get install -y gcc-arm-linux-gnueabihf  ;; \
-    && rm -rf /var/lib/apt/lists/*
+        "arm:v6")  apt-get install -y gcc-arm-linux-gnueabi    ;; \
+      esac; \
+    fi && \
+    rm -rf /var/lib/apt/lists/*
 
 # Map TARGETARCH + TARGETVARIANT → Rust target triple, then install it.
 RUN case "${TARGETARCH}:${TARGETVARIANT}" in \
@@ -24,8 +29,9 @@ RUN case "${TARGETARCH}:${TARGETVARIANT}" in \
     esac > /rust_target && \
     rustup target add "$(cat /rust_target)"
 
-# Tell Cargo which cross-linker to use for each foreign target.
+# Tell Cargo which cross-linker to use — only needed when cross-compiling.
-RUN mkdir -p /root/.cargo && cat >> /root/.cargo/config.toml <<'EOF'
+RUN if [ "${BUILDPLATFORM}" != "${TARGETPLATFORM}" ]; then \
+      mkdir -p /root/.cargo && cat >> /root/.cargo/config.toml <<'EOF'
 [target.aarch64-unknown-linux-gnu]
 linker = "aarch64-linux-gnu-gcc"
 
@@ -35,6 +41,7 @@ linker = "arm-linux-gnueabihf-gcc"
 [target.arm-unknown-linux-gnueabi]
 linker = "arm-linux-gnueabi-gcc"
 EOF
+    fi
 
 WORKDIR /build