Reorganise settings..

..including attempting to group options by likelihood of being edited,
removing some settings that weren't really configurable, and adding some
explanatory text

compose.yml

@@ -30,13 +30,13 @@ x-environment:
   - REAL_IP_FROM
   - REAL_IP_HEADER
   - RECIPIENT_DELIMITER
-  - REDIS_ADDRESS
+  - REDIS_ADDRESS=db
   - REJECT_UNLISTED_RECIPIENT
   - RELAYHOST
   - RELAYNETS
   - SECRET_KEY_FILE=/run/secrets/secret_key
   - SITENAME
-  - SUBNET
+  - SUBNET=192.168.203.0/24
   - TLS_CERT_FILENAME
   - TLS_FLAVOR
   - TLS_KEYPAIR_FILENAME
@@ -92,7 +92,7 @@ services:
         - "traefik.enable=true"
         - "traefik.docker.network=proxy"
         - "traefik.http.services.${STACK_NAME}.loadbalancer.server.port=80"
-        - "traefik.http.routers.${STACK_NAME}.rule=Host(`${WEB_DOMAIN}`)"
+        - "traefik.http.routers.${STACK_NAME}.rule=Host(`${DOMAIN}`)"
         - "traefik.http.routers.${STACK_NAME}.tls.certresolver=${LETS_ENCRYPT_ENV}"
         - "traefik.http.routers.${STACK_NAME}.entrypoints=web-secure"
         - "coop-cloud.${STACK_NAME}.version=1.0.2+2.0.23"
@@ -178,7 +178,7 @@ services:
         --dest /output --domain-subdir=true --version v2
         --post-hook "sh /usr/bin/certdumper_post.sh"'
     environment:
-      - DOMAIN=$WEB_DOMAIN
+      - DOMAIN=$DOMAIN
     volumes:
       # Folder, which contains the acme.json
       - type: volume