How to copy Docker images from one host to another without using a repository

Question

How can I move a Docker image from one machine to another without relying on a repository, private or public?

In VirtualBox, I develop my own image, which I then try to deploy to other PCs for real-world use.

It can't be reproduced from a Dockerfile because it's based on my own based image (like Red Hat Linux). My Dockerfile isn't as portable as I'd like it to be.

Is there anything I can do using basic commands? Or is there another option?