#157

Replace the hardcoded /var/www/html by a webroot named volume in docker-compose.yml.

In the Docker compose nginx service, the hardcoded /var/www/html was declared
as a volume which would cause issues when the /var/ was not available on the
local machine.

The Docker volume directory /var/lib/docker/volumes/dejacode_webroot/_data can
now be used in place of /var/www/html as the certbot --webroot-path for
HTTP certificate management.