Matrix trimming not working correctly

[mthalman]

Ever since merging dotnet/dotnet-buildtools-prereqs-docker#1572, matrix trimming has not been working correctly for the buildtools-prereqs repo. For every CI build, it includes many Dockerfile paths for the matrix that shouldn't be. Based on the log, it seems this is because it's not correctly computing the latest base image digest SHA for some images. For those images it is able to compute the base image digest, it's correctly trimming them from the matrix.

Example build (internal link)

[lbussell]

I see that in the pipeline logs some of the base image SHAs are not being resolved at all:

Sending HTTP request: https://***.azurecr.io/oauth2/token?service=***.azurecr.io&scope=repository:mirror/library/alpine:pull
Image info's base image digest SHA: sha256:ea71a031ed91cd46b00d438876550bc765da43b4ae40f331a12daf62f0937758
Latest base image digest SHA: 
Base image digests match: False
CACHE MISS

I'll investigate.