Update Dockerfile for multi-architecture support and enhance coding rules

- Modify Dockerfile to support multi-platform builds using TARGETARCH.
- Update coderule.mdc to enforce source code organization under the `src/` directory.
- Remove outdated rule from python.mdc regarding `src/` layout.

Dockerfile

@@ -1,7 +1,9 @@
-FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+ARG TARGETARCH
 WORKDIR /src
 COPY . .
-RUN dotnet publish -c Release -o /app
+RUN arch=$([ "$TARGETARCH" = "amd64" ] && echo "x64" || echo "$TARGETARCH") && \
+    dotnet publish -c Release -o /app --os linux --arch $arch
 
 FROM mcr.microsoft.com/dotnet/aspnet:10.0
 WORKDIR /app