Support multi-arch Docker builds

Dockerfile

@@ -1,16 +1,40 @@
-FROM rust:1-bookworm AS builder
+FROM --platform=$BUILDPLATFORM rust:1-bookworm AS builder
 
 WORKDIR /src
 RUN apt-get update \
-    && apt-get install --no-install-recommends -y build-essential ca-certificates \
+    && apt-get install --no-install-recommends -y \
+        build-essential ca-certificates \
+        gcc-aarch64-linux-gnu gcc-x86-64-linux-gnu \
+        libc6-dev-arm64-cross libc6-dev-amd64-cross \
+        linux-libc-dev-arm64-cross linux-libc-dev-amd64-cross \
     && rm -rf /var/lib/apt/lists/*
 
+ARG TARGETARCH
 COPY Cargo.toml Cargo.lock ./
 COPY migrations ./migrations
 COPY src ./src
 COPY tests ./tests
 
-RUN cargo build --release --locked
+RUN case "$TARGETARCH" in \
+        amd64) \
+            rust_target=x86_64-unknown-linux-gnu; \
+            linker=x86_64-linux-gnu-gcc; \
+            ;; \
+        arm64) \
+            rust_target=aarch64-unknown-linux-gnu; \
+            linker=aarch64-linux-gnu-gcc; \
+            ;; \
+        *) \
+            echo "unsupported TARGETARCH=$TARGETARCH" >&2; \
+            exit 1; \
+            ;; \
+    esac \
+    && rustup target add "$rust_target" \
+    && env "CARGO_TARGET_$(printf '%s' "$rust_target" | tr '[:lower:]-' '[:upper:]_')_LINKER=$linker" \
+        CC="$linker" \
+        cargo build --release --locked --target "$rust_target" \
+    && mkdir -p /out \
+    && cp "target/$rust_target/release/kosync-rs" /out/kosync-rs
 
 FROM debian:bookworm-slim
 
@@ -19,7 +43,7 @@ RUN apt-get update \
     && rm -rf /var/lib/apt/lists/* \
     && useradd --system --uid 10001 --home-dir /data --create-home kosync
 
-COPY --from=builder /src/target/release/kosync-rs /usr/local/bin/kosync-rs
+COPY --from=builder /out/kosync-rs /usr/local/bin/kosync-rs
 
 USER kosync
 WORKDIR /data