fix(docker): cross-compile API image without QEMU SDK

dotnet restore under emulated arm64 aborts on the amd64 runner;
build on BUILDPLATFORM and publish with -a TARGETARCH instead.

docker/Dockerfile.api

@@ -1,8 +1,16 @@
-FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+# syntax=docker/dockerfile:1
+# Build on the runner arch (amd64); publish for TARGETARCH (arm64 on Pi).
+# Avoids QEMU-emulated dotnet restore, which aborts under plugins/docker --platform.
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+ARG TARGETARCH
 WORKDIR /src
 COPY src/backend/ .
-RUN dotnet restore iLED.slnx
-RUN dotnet publish src/iLED.Api/iLED.Api.csproj -c Release -o /app/publish
+RUN dotnet restore iLED.slnx -a $TARGETARCH
+RUN dotnet publish src/iLED.Api/iLED.Api.csproj \
+    -c Release \
+    -o /app/publish \
+    -a $TARGETARCH \
+    --no-restore
 
 FROM mcr.microsoft.com/dotnet/aspnet:10.0
 WORKDIR /app