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/backend.csproj

@@ -39,7 +39,7 @@
     <Exec WorkingDirectory="$(SpaRoot)" Command="npm ci" />
   </Target>
 
-  <Target Name="PublishRunWebpack" AfterTargets="ComputeFilesToPublish">
+  <Target Name="PublishRunWebpack" AfterTargets="ComputeFilesToPublish" Condition=" '$(SkipSpaBuild)' != 'true' ">
     <!-- Build the frontend -->
     <Exec WorkingDirectory="$(SpaRoot)" Command="npm ci" Condition=" !Exists('$(SpaRoot)node_modules') " />
     <Exec WorkingDirectory="$(SpaRoot)" Command="npm run build" />