Cache result of Docker OS to speed up XUnit test discovery

[lbussell]

I got frustrated with slow Test Discovery and used PerfView on a run of our unit tests to see why it took so long. DockerOS was being accessed many times during test discovery because we have the custom LinuxImageTheory attribute. Each time it was accessed, it took two seconds to call docker.exe, even though the result never changes during a single test run.

Wrapping it in a Lazy<> speeds up test discovery by a factor of ~60.

Before - 27.64 seconds

Starting test discovery, please wait...
[xUnit.net 00:00:00.00] xUnit.net VSTest Adapter v2.4.5+1caef2f33e (64-bit .NET 9.0.1)
[xUnit.net 00:00:00.70]   Discovering: Microsoft.DotNet.Docker.Tests
[xUnit.net 00:00:28.34]   Discovered:  Microsoft.DotNet.Docker.Tests

After - 0.44 seconds

Starting test discovery, please wait...
[xUnit.net 00:00:00.00] xUnit.net VSTest Adapter v2.4.5+1caef2f33e (64-bit .NET 9.0.1)
[xUnit.net 00:00:00.66]   Discovering: Microsoft.DotNet.Docker.Tests
[xUnit.net 00:00:01.10]   Discovered:  Microsoft.DotNet.Docker.Tests