When you make changes to the files inside a Docker container, your changes are not picked up until the server is restarted. But at that point, the container is also restarted, thus discarding your changes…
The Docker Way Of Doing Things© is to fork edx-platform, apply your changes in your fork, and then re-build the openedx Docker image with your fork (Configuration and customisation — Tutor documentation).
The problem with the above approach is that you will have to rebase your changes with every upgrade, which is costly and error-prone. The Open edX Way Of Doing Things© (which is better) is to propose a change upstream, in edx-platform, to make it possible to customize the setting default value.