Fix dockerfile path

Update from .NET 6 to .NET 8.

Author: martincostello

docker/docker-compose-aspnetcore/docker-compose.self-contained.oats.yml

@@ -3,7 +3,7 @@ services:
     image: ${DOCKER_REGISTRY-}aspnetcore
     build:
       context: ../..
-      dockerfile: examples/net6.0/aspnetcore/Dockerfile
+      dockerfile: examples/net8.0/aspnetcore/Dockerfile
       args:
         DOTNET_PUBLISH_ARGS: "--self-contained true /p:PublishSingleFile=true"
     entrypoint: ./aspnetcore