Containers have layers. So if you create an instance of a syncthing container whoever built that container would have started with some other container. Alpine linux is a very popular base layer, just used as an example in this discussion.

When you download an image, all the layers underlying the application that you actually wanted, will only be as fresh as the last time the maintainer built that image. So if there were a bug in the alpine base, that might have been fixed in alpine, but wouldn’t by pushed through to whatever you downloaded.