Whereas Docker is focusing on containers for servers operated by “devops”, AppImage is focusing on desktop applications operated by end users.
Whereas Docker puts everything (besides the Kernel) into a container and shields it from the host, AppImages use the host OS and do not shield the application from it.
That being said, you could use AppImage for server applications, and you could bundle everything if you wanted. But then you could probably just as well use Docker.
Compare AppImages to .app applications inside .dmg disk images on macOS.