So, on the readme, there’s the explanation on how the image was created:
The docker image used was generated by OpenMRS SDK, and deployed to Docker hub from CI:
$ mvn org.openmrs.maven.plugins:openmrs-sdk-maven-plugin:3.7.0:build-distro -B -Ddir=demo-server -Ddistro=referenceapplication:<version> -Dbundled=true
$ cd demo-server/web/
$ docker build -t openmrs/openmrs-reference-application-distro:<version> .
$ docker push openmrs/openmrs-reference-application-distro:<version>
If you want to see how we deploy demo, you have here: https://github.com/openmrs/openmrs-contrib-ansible-docker-compose/tree/master/files/demo
Now, during build we just run that command, build the docker image and push.