I'm trying to understand the difference between images used in KVM / VMware / VirtualBox and Docker images + containers.
I can use i.e. an Ubuntu ISO to install the entire operating system into i.e. VirtualBox, essentially writing everything into an image file stored somewhere on the machine. Now, if for whatever reason I decide that I want to move the Ubuntu image and run it on a different computer, I can just copy the image to that other computer and run it from there.
Now, Docker also has images that may also have Ubuntu inside running pretty much any application. I have to setup a container that stores all my changes and states. Now, if I want to do the same thing as I did with the VirtualBox image, transfer my Docker image + container to another computer and run it from there, this step seems to be pretty difficult. I can't really understand why.
So my question is, what exactly is the conceptional difference between Docker images and the other ones mentioned above? Why can't I just copy those images around machines as I please?