Added new replacement variable for nginx-secure: ___NAMESERVER___. This
is parsed out from the system resolv.conf and used to resolve the
ip's of the containers dynamically
Modified nginx-secure.conf to resolve certbot dynamically
NOTE: if using `docker-compose run certbot ...`, set `--name certbot` so it
can be resolved properly
Added CMD /start.sh back in, guessing it was removed by accident
Docker got rid of legacy links so there are no longer env vars pointing
to other containers. Instead, we can reference the containers directly.
This fixes problems with let's encrypt related to missing env vars.
See notice at the top of
https://docs.docker.com/compose/link-env-deprecated/
