Update warning on Docker image to note connection permissiveness

installation/single_machine_docker.rst

@@ -6,7 +6,12 @@ Docker (Mac or Linux)
 This section describes setting up a Citus cluster on a single machine using docker-compose.
 
 .. note::
-   **The Docker image is intended for development/testing purposes only**, and has not been prepared for production use. Consider data in Docker instances to be ephemeral.
+   **The Docker image is intended for development/testing purposes only**, and
+   has not been prepared for production use. The images use default connection
+   settings, which are very permissive, and not suitable for any kind of
+   production setup. These should be updated before using the image for
+   production use. The PostgreSQL manual `explains how <http://www.postgresql.org/docs/current/static/auth-pg-hba-conf.html>`_ to
+   make them more restrictive.
 
 **1. Install Docker Community Edition and Docker Compose**