Allow for configuring volume mounts, ports, the site URL, and volume
mounts within the Docker Compose file using environment variables from
within and .env file.
This means that the same Docker Compose file could be used to generate
and serve the site locally using an anonymous volume, as well as
generate a one-time build into a local directory to be served by Apache
or Nginx on production.
Use Traefik as a local reverse proxy so that the site can be accessed
from http://oliverdavies.localhost rather than http://localhost:8000,
which is the default for Sculpin.
I've also overridden the command used for the app service so that it
exposes port 80 and serves the site on that port, as well as configuring
the URL to match within the generated output.
