Allow the generated Docker images to act as SSL endpoints

[bartfeenstra]

When the betty.plugin.nginx.Nginx plugin's https configuration is set to null or true, nginx configuration is generated with an SSL/HTTP2 server, which is invalid until someone manually adds ssl_* directives. Instead, warn users about this, and in Dockerfile install Let's Encrypt and automatically configure and renew the SSL certificate.

Definition of Done

• If the plugin's https configuration is set to true:
  • Install certbot and ensure it renews automatically
  • Request a certificate upon launch
• Introduce new email_address configuration for the certificate request.
• Update the Dockerfile documentation in README.md, and remove the warning about HTTPS not being support, and add instructions that is HTTPS is used by the plugin, the container opens port 443 in addition to port 80.