Either create a Dockerfile that is based on the original one as the other comment suggests, or find the git repo of the original Image, fork it and add your changes to the Dockerfile.

If you want to include changes from the original one you’ll need to set up some kind of regular job that checks for a new image or changes in the git repo and then applies your changes.

If you need it to be available at Dockerhub, ghcr or the like, you need some additional setup, usually a CI that pushes it.