What is Docker
I came across Docker a few months back, when I was looking for a way of managing app releases running on Raspberry Pi computers. If you know what a virtual machine is then you should not have a problem understanding what Docker is, but I like to think of it as VMs on steroids. I recommend you give the Docker website a visit and have a little look around.
Installing Docker CE
Traditionally Docker only runs on 64bit Intel/AMD machines, that will not work on the ARM based RPi single board computer. Fortunately, clever folk at Docker HQ released a version, which does run on ARM based platforms. To install it on your Raspberry Pi run the following commands in the terminal, I recommend you stick with the official jessie or stretch SD card images pulled directly from the Raspberry Pi website. You can either go for the standard release with GUI or the lite version, whatever floats your boat.
curl -fsSL get.docker.com -o get-docker.sh sudo sh get-docker.sh sudo usermod -aG docker pi sudo reboot