Multi-platform Docker build: amd64, arm64, armv7, armv6

Dockerfile now uses BuildKit TARGETARCH/TARGETVARIANT to pick the Rust
cross-compilation target automatically. The build stage always runs on
the host platform for speed.

Makefile provides named targets:
  make up-amd64   # Mac Intel / Linux desktop
  make up-arm64   # Mac M1/M2/M3, Pi 4/5 (64-bit OS)
  make up-armv7   # Pi 2/3/4 (32-bit OS)
  make up-armv6   # Pi Zero / Pi 1

infra/Dockerfile.server

@@ -1,12 +1,40 @@
+# syntax=docker/dockerfile:1
 # ── Build stage ───────────────────────────────────────────────────────────────
-FROM rust:1.94-slim-bookworm AS builder
+# Always run the compiler on the *build* host for speed; cross-compile to target.
+FROM --platform=$BUILDPLATFORM rust:1.86-slim-bookworm AS builder
 
+ARG TARGETARCH
+ARG TARGETVARIANT
+
+# Cross-compilation toolchains for every supported target.
 RUN apt-get update && apt-get install -y \
     pkg-config \
-    musl-tools \
+    gcc-aarch64-linux-gnu \
+    gcc-arm-linux-gnueabihf \
+    gcc-arm-linux-gnueabi \
     && rm -rf /var/lib/apt/lists/*
 
-RUN rustup target add armv7-unknown-linux-gnueabihf
+# Map TARGETARCH + TARGETVARIANT → Rust target triple, then install it.
+RUN case "${TARGETARCH}:${TARGETVARIANT}" in \
+      "amd64:")  echo x86_64-unknown-linux-gnu         ;; \
+      "arm64:")  echo aarch64-unknown-linux-gnu         ;; \
+      "arm:v7")  echo armv7-unknown-linux-gnueabihf     ;; \
+      "arm:v6")  echo arm-unknown-linux-gnueabi         ;; \
+      *)         echo x86_64-unknown-linux-gnu          ;; \
+    esac > /rust_target && \
+    rustup target add "$(cat /rust_target)"
+
+# Tell Cargo which cross-linker to use for each foreign target.
+RUN mkdir -p /root/.cargo && cat >> /root/.cargo/config.toml <<'EOF'
+[target.aarch64-unknown-linux-gnu]
+linker = "aarch64-linux-gnu-gcc"
+
+[target.armv7-unknown-linux-gnueabihf]
+linker = "arm-linux-gnueabihf-gcc"
+
+[target.arm-unknown-linux-gnueabi]
+linker = "arm-linux-gnueabi-gcc"
+EOF
 
 WORKDIR /build
 
@@ -14,12 +42,18 @@ WORKDIR /build
 COPY Cargo.toml Cargo.lock* ./
 COPY server/Cargo.toml ./server/
 RUN mkdir -p server/src && echo 'fn main(){}' > server/src/main.rs
-RUN cargo build --release --target armv7-unknown-linux-gnueabihf -p hiy-server 2>/dev/null || true
+RUN TARGET=$(cat /rust_target) && \
+    cargo build --release --target "$TARGET" -p hiy-server 2>/dev/null || true
 RUN rm -f server/src/main.rs
 
 # Build actual source.
 COPY server/src ./server/src
-RUN touch server/src/main.rs && cargo build --release --target armv7-unknown-linux-gnueabihf -p hiy-server
+RUN TARGET=$(cat /rust_target) && \
+    touch server/src/main.rs && \
+    cargo build --release --target "$TARGET" -p hiy-server
+
+# Normalise binary location so the runtime stage doesn't need to know the target.
+RUN cp /build/target/"$(cat /rust_target)"/release/hiy-server /usr/local/bin/hiy-server
 
 # ── Runtime stage ─────────────────────────────────────────────────────────────
 FROM debian:bookworm-slim
@@ -30,11 +64,10 @@ RUN apt-get update && apt-get install -y \
     curl \
     bash \
     python3 \
-    # Docker CLI (no daemon — uses host socket)
     docker.io \
     && rm -rf /var/lib/apt/lists/*
 
-COPY --from=builder /build/target/armv7-unknown-linux-gnueabihf/release/hiy-server /usr/local/bin/hiy-server
+COPY --from=builder /usr/local/bin/hiy-server /usr/local/bin/hiy-server
 
 WORKDIR /app
 

infra/Makefile

@@ -0,0 +1,35 @@
+# HIY — docker compose helpers
+# Usage: make <target>
+#
+# Supported platforms
+#   up-amd64   — x86-64 (Mac Intel, Linux desktop, CI)
+#   up-arm64   — ARM 64-bit (Mac M1/M2/M3, Pi 4 / Pi 5 running 64-bit OS)
+#   up-armv7   — ARM 32-bit v7 (Pi 2 / Pi 3 / Pi 4 running 32-bit OS)
+#   up-armv6   — ARM 32-bit v6 (Pi Zero, Pi 1)
+#
+# The default target builds for linux/amd64 (same as 'up-amd64').
+
+COMPOSE = docker compose
+UP      = $(COMPOSE) up --build
+
+.PHONY: up up-amd64 up-arm64 up-armv7 up-armv6 down logs
+
+up: up-amd64
+
+up-amd64:
+	PLATFORM=linux/amd64   $(UP)
+
+up-arm64:
+	PLATFORM=linux/arm64   $(UP)
+
+up-armv7:
+	PLATFORM=linux/arm/v7  $(UP)
+
+up-armv6:
+	PLATFORM=linux/arm/v6  $(UP)
+
+down:
+	$(COMPOSE) down
+
+logs:
+	$(COMPOSE) logs -f server

infra/docker-compose.yml

@@ -18,6 +18,7 @@ services:
 
   # ── Control plane ─────────────────────────────────────────────────────────
   server:
+    platform: ${PLATFORM:-linux/amd64}
     build:
       context: ..
       dockerfile: infra/Dockerfile.server