Refactor docker compose exec usage

Introduce centralized variables:
- docker_compose_command_base
- docker_compose_command_exec

Replaced hardcoded 'docker compose exec' with '{{ docker_compose_command_exec }}'
across multiple roles (BigBlueButton, EspoCRM, Friendica, Listmonk, Mailu, Matrix, OpenProject).
Ensures consistent environment file loading and reduces duplicated code.

Details: https://chatgpt.com/share/68d6a276-19d0-800f-839d-d191d97f7c41

roles/web-app-mailu/tasks/04_generate-and-read-dkim.yml

@@ -1,6 +1,6 @@
 - name: Check if DKIM private key file exists in the antispam container
   command: >
-    docker compose exec -T antispam
+    {{ docker_compose_command_exec }} -T antispam
     test -f {{ MAILU_DKIM_KEY_PATH }}
   register: dkim_key_file_stat
   failed_when:  false
@@ -10,7 +10,7 @@
 
 - name: Generate DKIM key
   command: >
-    docker compose exec -T antispam
+    {{ docker_compose_command_exec }} -T antispam
     rspamadm dkim_keygen -s dkim -d {{ MAILU_DOMAIN }} -k {{ MAILU_DKIM_KEY_PATH }}
   register: dkim_keygen_output
   when: dkim_key_file_stat.rc != 0
@@ -20,7 +20,7 @@
 
 - name: Fetch DKIM private key from antispam container
   shell: >
-    docker compose exec -T antispam
+    {{ docker_compose_command_exec }} -T antispam
     cat {{ MAILU_DKIM_KEY_PATH }}
   args:
     chdir: "{{ docker_compose.directories.instance }}"