buildbot-docker-example-config | example configuration for running buildbot | Continuous Deployment library

The docker example uses this file as the basis of the docker image: https://github.com/buildbot/buildbot/blob/master/master/Dockerfile

Which defines as its entrypoint this script: https://github.com/buildbot/buildbot/blob/master/master/docker/start_buildbot.sh

In that script the configuration is explicitly handled by downloading and extracting:

until curl -sL $BUILDBOT_CONFIG_URL | tar -xz --strip-components=1 --directory=$B/$BUILDBOT_CONFIG_DIR

and further linked as the master.cfg file. So all in all the magic depends on the additional script setting up the configuration file so it matches the typical setting.