Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -17,7 +17,7 @@
 # See other available tags for .NET Core: https://hub.docker.com/_/microsoft-dotnet
 # Learn more about Docker tags: https://docs.docker.com/glossary/?term=tag
 # and the Docker itself: https://opensource.com/resources/what-docker
-image: mcr.microsoft.com/dotnet/sdk:latest
+image: mcr.microsoft.com/dotnet/sdk:3.1
 
 # ### Define variables
 #