Have nitriding start star-randsrv

[NullHypothesis]

We currently use a shell script to start both star-randsrv and nitriding. Nitriding now supports the -appcmd command line switch, which instructs it to run the given executable. We should be able to get rid of start.sh and instead use the following command:

nitriding \
	-acme \
	-fqdn "star-randsrv.bsg.brave.com" \
	-appurl "https://github.com/brave/star-randsrv" \
	-appwebsrv "http://127.0.0.1:8080" \
        -appcmd "star-randsrv --epoch-seconds 604800" \
	-extport 443 \
	-intport 8081 &

With that in place, we no longer need a shell in our Docker image, and we might even be able to use a scratch image — provided that we add root CA certificates to talk to Let's Encrypt's infrastructure.