How to specify .NET version?

[jongalloway]

Since .NET Aspire includes support for current and previous .NET version (currently .NET 8 and .NET 9), this feature should allow specifying previous .NET version. After adding to dotnet/aspire-samples, I'm seeing that several samples specify .NET 8 and won't run without additional install for that version.

I see that .NET is specified using dependsOn dotnet. The best workaround in that specific case is to just also include the dotnet feature specifically and include an additionalVersion I think, but this seems like a common use case which would be important to include in the aspire-devcontainer feature.

Repro:

1. Open devcontainer on dotnet/aspire-samples
2. Execute ./build.sh
3. Execute dotnet run --project samples/AspireWithPython/AspireWithPython.AppHost/AspireWithPython.AppHost.csproj

Output:

Using launch settings from samples/AspireWithPython/AspireWithPython.AppHost/Properties/launchSettings.json...
Building...
You must install or update .NET to run this application.

App: /workspaces/aspire-samples/samples/AspireWithPython/AspireWithPython.AppHost/bin/Debug/net8.0/AspireWithPython.AppHost
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
.NET location: /usr/share/dotnet

The following frameworks were found:
  9.0.2 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  9.0.3 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

Learn more:
https://aka.ms/dotnet/app-launch-failed

To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=8.0.0&arch=x64&rid=linux-x64&os=debian.12