Note about official Docker images

doc/development/design/index.mdwn

@@ -322,6 +322,7 @@
         + Scenario 2: Tests need PostgreSQL
             + Alternative 1: Use existing PostgreSQL on system.
             + Alternative 2: Use Docker <https://hub.docker.com/_/postgres/>.
+                + Note: similar to official VMs, there can be a concept of official Docker images.
             + Alternative 3: If already using Docker for running the tests separately, use Docker Compose.
             + Requires: connection string (host, port, auth)
 - Testing of Orb database