build: optimize Docker build by skipping redundant SPA build

Set SkipSpaBuild environment variable in Dockerfile to prevent MSBuild from rebuilding the frontend during publish, as it is already built in a separate stage. Also disable NuGet audit warnings to reduce build noise.

backend/Dockerfile

@@ -11,11 +11,14 @@ FROM mcr.microsoft.com/dotnet/sdk:9.0 AS backend-build
 WORKDIR /src
 COPY nuget.config ./
 COPY backend.csproj ./
+ENV DOTNET_NUGET_AUDIT=false
 RUN dotnet nuget list source
 RUN dotnet restore
 COPY . .
 # Copy the frontend build output from the previous stage
 COPY --from=frontend-build /app/dist ./ClientApp/dist
+# Skip the SPA build in MSBuild since it's already built in Stage 1
+ENV SkipSpaBuild=true
 RUN dotnet publish -c Release -o /app/publish
 
 # Stage 3: Runtime