I got Docker working properly since my last log entries. I just had to install Docker Toolbox, uninstall Docker Toolbox when it stopped working, uninstall VirtualBox too, and reinstall Docker Toolbox, letting it install VirtualBox.

Therefore, I have a tip: If you're going to use Docker Toolbox with VirtualBox as the backend, uninstall VirtualBox if you have it installed, and let the Docker Toolbox installer reinstall it. It will be much more reliable that way.

Another solution I tried was Play with Docker, which a web-based Docker learning environment. That worked great at first, but that was in the middle of the night when nobody else was using it. During the day (of North America) I found I couldn't get a session because it was fully loaded. So I just used my local Docker, and that was fine.