Use prebuilt docker images instead of installing dependencies on every run

[dibarbet]

No description provided.

[Copilot]

Pull request overview

This PR updates the Azure Pipelines Linux test stages to rely on prebuilt container images that already contain required runtime prerequisites, removing the previous “install extra Linux dependencies inside the container” flow.

Changes:

• Removed the installAdditionalLinuxDependencies parameter wiring and the test-linux-docker-prereqs.yml template usage.
• Switched Linux test stages to use mcr.microsoft.com/dotnet-buildtools/prereqs:* images instead of mcr.microsoft.com/dotnet/sdk:*.
• Deleted the no-longer-used azure-pipelines/test-linux-docker-prereqs.yml template.

Show a summary per file

File	Description
azure-pipelines/test.yml	Removes conditional Linux docker prereq install step and related parameter plumbing.
azure-pipelines/test-matrix.yml	Removes the now-unused installAdditionalLinuxDependencies parameter passthrough.
azure-pipelines/test-linux-docker-prereqs.yml	Deletes the template that installed extra VS Code runtime deps in the container via docker exec.
azure-pipelines.yml	Updates Linux stages to use prebuilt prereqs container images for .NET 8–11 test runs.

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Suppressed comments (3)

azure-pipelines.yml:109

• The container image is referenced by a mutable tag. For build reproducibility and supply-chain safety, consider pinning the image to an immutable digest (e.g., ...@sha256:<digest>) so the CI environment can’t change unexpectedly.

      containerName: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-24.04-net9.0-vscode-csharp-amd64

azure-pipelines.yml:124

• The container image is referenced by a mutable tag. For build reproducibility and supply-chain safety, consider pinning the image to an immutable digest (e.g., ...@sha256:<digest>) so the CI environment can’t change unexpectedly.

      containerName: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-24.04-net10.0-vscode-csharp-amd64

azure-pipelines.yml:139

• The container image is referenced by a mutable tag. For build reproducibility and supply-chain safety, consider pinning the image to an immutable digest (e.g., ...@sha256:<digest>) so the CI environment can’t change unexpectedly.

      containerName: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-26.04-net11.0-vscode-csharp-amd64

• Files reviewed: 4/4 changed files
• Comments generated: 3
• Review effort level: Lite