dotnet sample app needs a default entrypoint or README

[arturenault]

The overall readme for sample apps says the sample apps can be built and run with:

pack build -p apps/<APP> --builder cnbs/sample-builder:<bionic OR alpine> sample-app
After building the app you should be able to simply run it via docker run -it -p 8080:8080 sample-app. Go to localhost:8080 to see the app running.

For the dotnet sample the build step works. However, docker run fails with:

❯ docker run -it -p 8080:8080 sample-app-dotnet
ERROR: failed to launch: determine start command: when there is no default process a command is required

The sample app should be fixed so it doesn't require a start command, or the dotnet sample should have its own readme indicating the correct command to start with. I've been digging and haven't been able to figure it out yet. The usual suspects ("dotnet" "dotnet run", etc) don't work.

[jromero]

cc: @micahyoung