Attempting to pull image-builder now results in auth error

[dagood]

Steps to reproduce the issue

1. Run a build that uses docker-tools ImageBuilder

Or:

1. Run docker pull mcr.microsoft.com/dotnet-buildtools/image-builder:2986591 locally

Expected behavior

Get the image.

Actual behavior

Error response from daemon:
Head "https://mcr.microsoft.com/v2/dotnet-buildtools/image-builder/manifests/2986591":
unauthorized:
{"errors":[
{"code":"UNAUTHORIZED","message":"authentication required, visit https://aka.ms/acr/authorization for more information. CorrelationId: 779101c9-808e-4d8b-8f2f-c515aabe04bd"}
]}`

(Newlines mine.)

Additional information (e.g. issue happens only occasionally)

Seems to have started between: https://dev.azure.com/dnceng/internal/_build/results?buildId=3001026&view=results and https://dev.azure.com/dnceng/internal/_build/results?buildId=3000998&view=results.

So, between 2:31 PM and 3:04 PM (Pacific).

I was running a somewhat large number of builds to debug some flakiness at the time.

[lbussell]

Does not seem to be restricted to image-builder:

> docker pull mcr.microsoft.com/dotnet-buildtools/image-builder:2986591
Error response from daemon: error from registry: authentication required, visit https://aka.ms/acr/authorization for more information. CorrelationId: 181aa587-a259-4f0d-addf-6a5d38db5732

[dagood]

Looks like this is an MCR outage. Sorry for the false(-ish) alarm.

https://portal.microsofticm.com/imp/v5/incidents/details/818344213/summary