Define a docker image in 10 bullets points

Docker image is a collection of filesystems. Each image has a unique UUID assigned via sha-256 algorithm.
Each image and a container has storage device assocaited with it.
Each file system represents a layer in the image.
The base layer for any image is the rootFS.
Every layer comes bundled with it’s parent layer.
All the layers of an image are read only.
At the time of container start, a merged layer is created including all the layers of corresponding image and made writable for custimization. Any change to this merged layer, doesn’t affect the actual image layers.
A container is one mounted instance of an image. As many containers from an image will be created, that many merged layers will be created for each container with a unique UUID.
The top layer has the highest precedence i.e. in case of duplicate files in multiple layers, the topmost layers file will be final output in the merged layer.
The default base directory for docker images and containers is /var/lib/docker.
When a container is stopped or killed, the corresponding merged layer will be cleaned from /var/lib/docker folder.