x.509/Makefile-stunnel: Add a basic Makefile for creating self-signed certs
x.509/Makefile-stunnel: Add a basic Makefile for creating self-signed certs

This is similar to Makefile-server, but it only creates a single
key/certificate pair.  It also bundles the private key and certificate
together in stunnel.pem, which is the format the stunnel image expects
(mount it at the container's /etc/stunnel/stunnel.pem).

fr34k8/dockerfile,wking/dockerfile,ostree/dockerfile