diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -1,6 +1,10 @@
 # Multiple Openfires in Docker
 This sets up multiple Openfire servers with associated PostgreSQL DBs in Docker containers for local testing.
 
+It relies on an official Postgres Docker image (from Docker Hub) and a local Openfire image, these are referred to in `docker-compose.yml`. The Openfire Docker image can be built using the `Dockerfile` in the root of the Openfire repository (https://github.com/igniterealtime/Openfire):
+
+    docker build -t openfire:fmuc .
+
 Running `./start.sh` will perform some cleanup then start the containers. When running, the system looks like this:
 
 ```