Explain default environment variables

docs/docker/l3d/drupal.md

@@ -50,6 +50,38 @@ that you get similar (but not identical) setup on all stages. It should not
 be identical, because you don't want all the development tools on your 
 production site.
 
+## Recommended global settings for your host
+
+Before starting with Drupal projects and L3D we recommend to create or edit an `.env`
+file in your user's home directory with the following content:
+
+```shell
+COMPOSER_DOWNGRADE=0
+DOCKER4DRUPAL_LIVE_HOST_USERNAME=YourUserName
+DOCKER4DRUPAL_TRAEFIK_CERT=fullchain.pem
+DOCKER4DRUPAL_TRAEFIK_DOMAIN=docker.localhost
+DOCKER4DRUPAL_TRAEFIK_KEY=privkey.pem
+DOCKER4DRUPAL_TRAEFIK_PORT=8000
+DOCKER4DRUPAL_TRAEFIK_PORTAINER=0
+DOCKER4DRUPAL_TRAEFIK_PORTS=8443
+DOCKER4DRUPAL_TRAEFIK_USESSL=0
+DORGFLOW=0
+DRUPAL_LIVE=0
+MAILHOG_HOST=smtp.freesmtpservers.com
+MAILHOG_MECHANISM=NONE
+MAILHOG_PASSWORD=
+MAILHOG_PORT=25
+MAILHOG_USERNAME=
+PHP_DEBUG=0
+PHP_VERSION=7.4
+WEBSERVER_CONFIG_OVERWRITE=0
+```
+
+Those will be used as default values for all your projects, and you may adjust
+them towards your own needs. With those defaults in place, you will then get less
+questions when setting up new projects and the local `.env` file will then only
+contain those values that are different form your global default.
+
 ## Working with multiple projects in parallel
 
 The composer plugin [Docker4Drupal](/composer/plugin/d4d) in combination with