trust the reverse proxy by default

roles/matrix-etherpad/defaults/main.yml

@@ -22,7 +22,12 @@ matrix_etherpad_user_gid: '5001'
 matrix_etherpad_container_http_host_bind_port: ''
 
 # A list of extra arguments to pass to the container
-matrix_etherpad_container_extra_arguments: []
+#
+# We assume that a reverse proxy is used and tell the container to trust it
+#   Details: https://github.com/ether/etherpad-lite/blob/develop/doc/docker.md
+matrix_etherpad_container_extra_arguments: [
+  '--env TRUST_PROXY=true'
+]
 
 matrix_etherpad_public_endpoint: '/etherpad'