Don't hardcode domains

roles/docker/defaults/main.yml

@@ -6,6 +6,7 @@ services:
   ### Internal services ###
   postfix:
     file: postfix.yml
+    domain: "smtp.{{ base_domain }}"
     version: "v3.5.0"
 
   nginx_proxy:

roles/docker/tasks/services/postfix.yml

@@ -17,4 +17,4 @@
     env:
       # Get all services which have allowed_sender_domain defined
       ALLOWED_SENDER_DOMAINS: "{{ services | dict2items | selectattr('value.allowed_sender_domain', 'true') | map(attribute='value.domain') | join(' ') }}"
-      HOSTNAME: "smtp.{{ base_domain }}" # the name the smtp server will identify itself as
+      HOSTNAME: "{{ services.postfix.domain }}" # the name the smtp server will identify itself as