add ability to use SSL in nginx

This adds the ability to volume mount a certificate and key, point a couple of environment variables at them, and have nginx serve both http and https sites.  The README has been updated with an example of how to do that.  It seems to be fairly robust, in that, if you stop the container, delete cert files, and restart the container, it enables just the http site.  But, if you stop it, put the cert files back (or replace them with others), then restart, the ssl site is back.