Update devcontainer to support .NET 9 and .NET 10

.devcontainer/Dockerfile

@@ -0,0 +1,11 @@
+FROM mcr.microsoft.com/dotnet/sdk:10.0
+
+# Install .NET 9 SDK side-by-side so the container supports both net9.0 and net10.0 builds.
+RUN set -eux; \
+    apt-get update; \
+    apt-get install -y --no-install-recommends curl ca-certificates; \
+    rm -rf /var/lib/apt/lists/*; \
+    curl -fsSL https://dot.net/v1/dotnet-install.sh -o /tmp/dotnet-install.sh; \
+    chmod +x /tmp/dotnet-install.sh; \
+    /tmp/dotnet-install.sh --channel 9.0 --quality ga --install-dir /usr/share/dotnet --skip-non-versioned-files; \
+    rm -f /tmp/dotnet-install.sh

.devcontainer/devcontainer.json

@@ -1,3 +1,7 @@
 {
-    "image": "mcr.microsoft.com/dotnet/sdk:8.0"
-}
+    "name": "csharp",
+    "build": {
+        "dockerfile": "Dockerfile",
+        "context": "."
+    }
+}