Docker image

Docker Image is a collection of multiple Filesystems.
Each docker Image consist of 1 Root FS + 1 User FS which forms the Base Image and it can have multiple number of application specific FS.
Each of this Filesystem corresponds to a layer, and each layer are identified by a unique Layer ID.
Each of the Layers have a corresponding Parent layer except the root layer.
When a container is created, all of the layers defined in the images are merged into a single Mount which is then attached to the runtime user.
Once the container is running , any file related changes made into the container gets recorded in two location diff and merged.
diff directory is persistent, while the merged directory is removed when the container is stopped/killed.
If the stopped container is started back again a new merged is created from the layers defined in image and the changes from the diff directory is applied to that merged and is then mounted to the runtime user.
Docker Root Dir can be located by using docker info command.
Docker uses Sha2-256 algorithm to hash and manage the FS.
Depending on the Storage driver, the FS are created in the corresponding storage driver's name folder e.g. overlay2 under docker root directory.
The Meta data related to the container is stored under Image directory.

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