Update Dockerfile for multi-stage .NET build process

Implemented a multi-stage build in the Dockerfile, adding a build stage with the .NET SDK for restoring and publishing the application. Introduced a runtime stage using the ASP.NET image, created a non-root user for enhanced security, and set necessary environment variables. The published output is now copied from the build stage, and the entry point is configured to launch the application.

Dockerfile

@@ -0,0 +1,35 @@
+# =========================
+# Build stage
+# =========================
+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
+ARG BUILD_CONFIGURATION=Release
+WORKDIR /src
+
+# Copy everything (simple + reliable for single-project Razor Pages apps)
+COPY . .
+
+# Restore & publish (self-contained trimming can be added later if desired)
+RUN dotnet restore
+RUN dotnet publish -c $BUILD_CONFIGURATION -o /app/publish --no-restore
+
+# =========================
+# Runtime stage
+# =========================
+FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS final
+WORKDIR /app
+
+# (Optional) Create non-root user for better security
+RUN useradd -m appuser
+ENV ASPNETCORE_URLS=http://+:8080 \
+    ASPNETCORE_ENVIRONMENT=Production \
+    DOTNET_RUNNING_IN_CONTAINER=true
+EXPOSE 8080
+
+# Copy published output
+COPY --from=build /app/publish ./
+
+# Switch to non-root
+USER appuser
+
+# Start the Razor Pages app
+ENTRYPOINT ["dotnet", "SimpleIdp.dll"]