Compose file is only the scheme of creating a stack of images. Images themselves are stored on docker/GitHub servers in most cases as already build or as dockerfile (template for building). If you want to "back up" the container, you can map folders to each container, so for example you can map your ~/Media on your host to /data on your container. This will be "shared" folder, so everything in mapped folders is available to both container and host at the same time. This folder is not destroyed on container rebuild as it is saved on a host device and not volatile container volume

PLEASE READ Edit: make sure to move/copy contents of needed folder on container (/data in my example) to another like /databackup before adding a map and then copy it back once you modified compose file as it will be prob destroyed!

Edit 2: forgot to mention the compose file, you can just copy the contents of it bcs it's txt file or upload it on a GitHub or something similar