Point a docker installation to a private git repo

Hi Discouse devs,

We are building a customized discourse using the docker installation, and would like to point to a private repo of ourselves. We are wondering what is the right of doing so, and what would be the best practice of pulling discourse updates in the future.

Right now, I was able to make it point to our repo by following instruction here: https://meta.discourse.org/t/how-to-install-my-customized-repo-of-discourse-on-production-server/23364, but because our repo is private, I had to hardcode the user name and password in the git repo url in the config file. I hope there is better way of doing so, and a standard way of getting discourse updates.