git pull would update the local copy of discourse_docker, not Discourse itself. The rebuild then pulls down the latest changes.
There’s an existing topic on an official docker image and the use of compose:
Please note that unless it becomes official, the only supportable install is the standard install method you’ve already followed. Third-party packages and stuff which sidesteps launcher is done at own risk and with no real support. As mentioned in the topic above some of the big changes such as upgrades to postgres haven’t been factored in as they haven’t been necessary since people have begun to experiment.