WIP: Generate HTTPS certs using Certbot

[shu8]

A lot of this has come from github.com/JonasAlfredsson/docker-nginx-certbot/:

• Update NGINX Dockerfile to install certbot via Snap, following these instructions: https://certbot.eff.org/lets-encrypt/ubuntufocal-nginx
• Update NGINX config to use certificates from letsencrypt output dir
• Update NGINX config to allow /.well-known/acme-challenge requests and route to letsencrypt directory
• Update run.sh to call certbot to generate/renew certificates -- note this loops every 60 seconds; maybe we want to increase the interval?