Special case scratch base image

[tofay]

If a container image has a image.base.ref.name of scratch then determine that there is no base image, rather than trying to pull "scratch".

In v1.1.9 component-detection tries to pull the "scratch@" image, which fails causing the following log to be displayed:

 [INFO] Base image scratch@ could not be found locally and could not be pulled. Results will not be mapped to base image layers 

In v1.2.4 component-detection still tries to pull the "scratch@" image but this time the TryPullImageAsync call at

component-detection/src/Microsoft.ComponentDetection.Detectors/linux/LinuxContainerDetector.cs

Line 225 in 0319816

await DockerService.TryPullImageAsync(refWithDigest, cancellationToken)))

must be succeeding, as component-detection doesn't omit that log, but subsequently hits an error like

[WARN] Scanning of image sha256:26f452efc71ea50bb61043de65c4fbbfcda3677ca6f013690898bfd286f22081 failed with exception: Object reference not set to an instance of an object. 

Looking at the changes between those releases, possibly the bump of Docker.Dotnet from 3.125.4 to 3.124.5 caused this change of behaviour. I can't verify that easily though as I'm hitting dotnet/Docker.DotNet#554, hence the bump to Docker.Dotnet to 3.125.10

Anyway, it seems sensible to special case scratch in this case.

[grvillic]

@tofay - Thanks for the contribution! Would you mind adding a test for this change to prevent regressions?

[tofay]

@tofay - Thanks for the contribution! Would you mind adding a test for this change to prevent regressions?

Added a UT for this.