Here’s how it works:
The Dockerfiles are in these github repositories.
A github action (or someone manually, following instructions like these) runs docker build in order to create a “Docker Image”. The image is uploaded to DockerHub.
When you install an extension in BlueOS, it pulls the image from Dockerhub, and stores it locally.
With the local image, it can instantiate new containers as it wishes.
What you need to do is to fork the ROS extension, add your new scripts, build and deploy a new image (I recommend using this action). Then you can just change the installed extension to use your new dockerhub remote and tags.