Update dockerfile.sample.txt image (#474)

- update docker sample image to use mcr registry

Author: embetten

samples/dockerfile.sample.txt

@@ -1,7 +1,7 @@
 # This sample is for Artifacts Credential Provider version 0.1.28 and it is incompatible with greater versions due to the .NET Core version this sample uses.
 
 # downloading the dotnet sdk image. Could be any docker sdk image with sdk > 2.1.500
-FROM microsoft/dotnet:2.1-sdk AS dotnet-builder
+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS dotnet-builder
 ARG FEED_URL
 ARG PAT