I have 3 environment specific appsettings files in my .Net core application
in project.json I have setup publishOptions
Honestly I think that is not the right task for a build pipeline. Also the publishing features of the dotnet cli are very limited. Go to external tools like Tseng showed you. Deployment is another domain with an own set of complexity than building.
There is not a build in way on dotnet cli beyond using external tools!