Add NuGet.config to test projects to fix nuget restore under network isolation

[Copilot]

nuget restore in test container builds fails under 1ES Pipeline Template network isolation because there's no NuGet.config in the Docker build context — NuGet falls back to api.nuget.org which is blocked.

• Added NuGet.config pointing to the Azure DevOps public feed (pkgs.dev.azure.com/dnceng/public/...) to all 8 test project directories that run nuget restore:
  • webapp-4.6.2, webapp-4.7, webapp-4.7.1, webapp-4.7.2, webapp-4.8, webapp-4.8.1
  • dotnetapp-4.8, dotnetapp-4.8.1

Each Dockerfile already does COPY . ./, so NuGet picks up the config automatically — no Dockerfile changes needed. Config content matches the repo root NuGet.config.

Original prompt

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This section details on the original issue you should resolve

<issue_title>NuGet restore blocked by network isolation in VerifyImagesWithWebApps Test</issue_title>
<issue_description>Pipeline: dotnet-framework-docker-official microsoft/dotnet-framework-docker#2900408

The Test stage fails across all 4 OS variants with the same root cause: nuget restore inside the test container cannot reach api.nuget.org due to 1ES Pipeline Template network isolation.

Failed Jobs

Job	OS	Test
Windows1809_amd64	ltsc2019	VerifyImagesWithWebApps (4.8)
Windows2022_amd64	ltsc2022	VerifyImagesWithWebApps (4.8.1)
Windows2025_amd64	ltsc2025	VerifyImagesWithWebApps (4.8.1)
WindowsLtsc2016_amd64	ltsc2016	VerifyImagesWithWebApps (4.8)

Root Cause

The webapp-4.8 test project Dockerfile runs nuget restore at build time, but the Docker build context (tests/.../projects/webapp-4.8/) does not include a NuGet.config. The container falls back to the default NuGet configuration pointing to https://api.nuget.org/v3/index.json.

The repo root NuGet.config uses the Azure DevOps public feed (pkgs.dev.azure.com/dnceng/public/...), which would work under network isolation, but it is not available inside the test container's Docker build context.

Packages Affected

From tests/.../projects/webapp-4.8/packages.config:

• Microsoft.Net.Compilers 2.6.1
• Microsoft.CodeDom.Providers.DotNetCompilerPlatform 1.0.8

Suggested Fix

Copy or generate a NuGet.config into the webapp test project build context that uses the internal Azure DevOps feed (https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json) instead of relying on the default api.nuget.org source.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes NuGet restore blocked by network isolation in VerifyImagesWithWebApps Test #1292

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