No problem, but a question related to Docker and Home Assistant

I am using Home Assistant Supervised. Every new release, I make some small changes to the HA image and commit that to an image with a new name. This seems to work fine for me; after a reboot, the new image seems to load, instead of the released image. However I never specifically run the new image, and don’t understand if Docker is responsible for starting the latest version of the image, or is it the Supervisor that knows there’s a new version and pick it up at startup? Any information/explanation is appreciated.