Add the possibility to pass extra flags to the docker container

roles/matrix-mailer/defaults/main.yml

@@ -9,6 +9,9 @@ matrix_mailer_docker_image: "devture/exim-relay:4.91-r3-0"
 matrix_mailer_container_user_uid: 100
 matrix_mailer_container_user_gid: 101
 
+# A list of extra arguments to pass to the container
+matrix_mailer_container_extra_arguments: []
+
 matrix_mailer_sender_address: "matrix@{{ matrix_domain }}"
 matrix_mailer_relay_use: false
 matrix_mailer_relay_host_name: "mail.example.com"