@Peter T Okay. I'll bite. What's a docker image?
In a nutshell, Docker is a virtualized Linux kernel. Unlike a VM, it relies on the host system for IO etc. It is available on many platforms such as Mac and Windows and is supported by most of the major vendors. And it is very lightweight.
A Docker image contains a Linux kernel that has been configured to run a particular application such as MySQL. This means it includes not only the kernel, but also all third party libraries etc that are required to run the application.
There is a lot more to Docker, but the part we are interested in here is the ability to quickly get an instance of a database server up and running on a local machine. Once Docker is installed, it requires just two commands: One to download an image from the vendor and another to start the image and map it to a local port.
Of course this means you can quickly test various database versions, clone instances and make changes etc..