Container tests need a better way of using earlier Runtime versions

[baronfel]

Echoing this - the container tests are in kind of an odd space (at least for the preview timeframe). The SDK version being used to build containers carries along a certain RuntimeFrameworkVersion implicitly, but the containers made available on mcr.microsoft.com lag behind in terms of the runtimes installed on them (by design - they are only made in the run-ups to each preview release) and so in the SDK we tend to make framework-dependent apps that we actually can't run in containers. We (containers team) need a better strategy for this and so I added these properties to 'pin' the runtimes for projects targeting net8.0 as a temporary measure.

Originally posted by @baronfel in #30038 (comment)

We (@dotnet/sdk-container-builds-maintainers) need some kind of testing strategy that is resilient to this timing issue.

[tmds]

I assumed this might be possible using RuntimeFrameworkVersion by setting it to a floating version, but no:

sdk/8.0.100-rc.2.23428.8/NuGet.targets(156,5): error : '[8.0.*-*]' is not a valid version string.

[tmds]

Some things which are possible:

<TargetFramework>net8.0</TargetFramework>
<RuntimeFrameworkVersion>8.0.0-preview1</RuntimeFrameworkVersion>

or

<TargetFramework>net8.0</TargetFramework>
<RuntimeFrameworkVersion>6.0.0</RuntimeFrameworkVersion>
<RollForward>Major</RollForward>