fix(docker-dev): pin the build stage to linux/amd64 so Grpc.Tools protoc runs

Phase 3 added an in-repo .proto compiled by Grpc.Tools; its bundled linux_arm64 protoc
segfaults (exit 139) on Apple-Silicon Docker, so the native-arm64 build stage could not
generate the gRPC stubs. dotnet publish is framework-dependent (portable IL + every RID's
native assets), so pinning ONLY the build stage to linux/amd64 leaves the runtime image
native arm64 (fast) while the one-time build is emulated. Matches the Dockerfile's stated
'targeting linux-x64' intent.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

docker-dev/Dockerfile

@@ -3,7 +3,12 @@
 # Compose drives OTOPCUA_ROLES + Cluster:* env per container to differentiate them.
 # A separate `migrator` stage (below) applies EF migrations once on bring-up.
 
-FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+# Build stage is PINNED to linux/amd64. Per-cluster mesh Phase 3 added an in-repo .proto compiled by
+# Grpc.Tools, whose bundled linux_arm64 `protoc` segfaults (exit 139) on Apple-Silicon Docker — so a
+# native-arm64 build stage cannot generate the gRPC stubs. `dotnet publish` here is framework-dependent
+# (portable IL + every RID's native assets), so the amd64-built output runs unchanged on the native
+# runtime image below; only this one-time build is emulated, runtime stays native/fast.
+FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/sdk:10.0 AS build
 WORKDIR /src
 COPY . .
 RUN dotnet restore ZB.MOM.WW.OtOpcUa.slnx