The author primary signature's timestamp found a chain building issue: UntrustedRoot: self signed certificate in certificate chain

Update: Check this announcement: https://github.com/NuGet/Announcements/issues/49

At the moment the issue appears to be related to the Debian image.

Switch to an Ubuntu or Alpine based image instead:

FROM mcr.microsoft.com/dotnet/sdk:5.0-focal AS build-env

Follow https://github.com/NuGet/Home/issues/10491 for updates.

In the Dockerfile file, I changed from

FROM mcr.microsoft.com/dotnet/aspnet:5.0-buster-slim

to

FROM mcr.microsoft.com/dotnet/sdk:5.0-alpine

This worked for me!

As already mentioned the current updates of issue could be followed here:
https://github.com/NuGet/Home/issues/10491
To shorten your journey
Known workarounds include:

• Downgrade to .NET Core 3.1
• If using docker, change your base image from "FROM mcr.microsoft.com/dotnet/sdk:5.0" to " FROM mcr.microsoft.com/dotnet/sdk:5.0-focal" or " FROM mcr.microsoft.com/dotnet/sdk:5.0-alpine"
• Put this inside the tag in your nuget.config to disable validation altogether (works with dotnet restore) :

<config> <add key="signatureValidationMode" value="accept" />

You can also check the status here: https://status.nuget.org/