You do not want to run your own version of Discourse from your own github repo. I promise. For many of the reasons that you have pointed out. You want to develop a plugin. Maybe you should see Beginners Guide to Install Discourse for Development using Docker and Beginner's Guide to Creating Discourse Plugins - Part 1
See also Can Discourse ship frequent Docker images that do not need to be bootstrapped?. Bitnami is out of date and not supported here.