I have 3 environment specific appsettings files in my .Net core application
in project.json I have setup publishOptions
You need to actually add the environment variables, according the official tutorial:
var builder = new ConfigurationBuilder()
.SetBasePath(env.ContentRootPath)
.AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: false, reloadOnChange: true)
// do not forget to add environment variables to your config!
.AddEnvironmentVariables();