Error pushing images in unofficial pipeline

[mthalman]

After merging the changes from #1498, the unofficial pipeline is having issues pushing images to the ACR.

The push refers to repository [msdotnetcontainersstagingtest.azurecr.io/build-staging/2820655/dotnet-buildtools/prereqs]
dce31c8f66ec: Preparing
aced4b97acad: Preparing
dfd600ff96b3: Preparing
110c184316c3: Preparing
4f3100c431d6: Preparing
3bd9ff297557: Preparing
3bd9ff297557: Waiting
dfd600ff96b3: Retrying in 5 seconds
dfd600ff96b3: Retrying in 4 seconds
dfd600ff96b3: Retrying in 3 seconds
dfd600ff96b3: Retrying in 2 seconds
dfd600ff96b3: Retrying in 1 second
received unexpected HTTP status: 500 Internal Server Error

Example build (internal link)

cc @lbussell

[lbussell]

I kicked off a build of the docker-tools-imagebuilder-unofficial pipeline to see if it reproduces there, to try and narrow down the issue.

[lbussell]

It didn't repro in https://dev.azure.com/dnceng/internal/_build/results?buildId=2822383&view=results.

It also seems to have worked in https://dev.azure.com/dnceng/internal/_build/results?buildId=2822352&view=logs&j=feee8b49-980c-5b7d-9f18-8080ca99e253&t=3630236d-f626-5024-f4c6-37277917578a.