Teach nitriding how to spawn the enclave application #58
Description
So far, we've been starting nitriding and the enclave application via a shell script that starts nitriding in the background, and then proceeds to start the enclave application. That works just fine but it requires a shell, which is why we often base enclave images on Alpine. Alpine is a heavy dependency just for a shell.
We could work around this by teaching nitriding how to spawn (and subsequently monitor) the enclave application in the background, so we don't need a shell any more. This would allow for much simpler and smaller enclave images: all we need is a statically-compiled nitriding and enclave application.
Ideally, nitriding would terminate when it detects that the enclave application terminated. This has the added benefit that our Kubernetes pod is then also going to shut down. In our current shell script-based approach, the Kubernetes pod continues if nitriding terminated because it's a background process.