Point base image to Microsoft Container Registry

As per:
https://azure.microsoft.com/blog/microsoft-syndicates-container-catalog/

Use the updated URL schema for .NET Core images

.gitlab-ci.yml

-image: microsoft/dotnet:latest
+image: mcr.microsoft.com/dotnet/core/sdk:latest
 
 stages:
     - build
@@ -15,4 +15,4 @@ build:
 test:
     stage: test
     script: 
-        - "dotnet test"
\ No newline at end of file
+        - "dotnet test"