Exploring Docker Image Layers and Size Management
Docker images are built from layers, where each layer represents a set of filesystem changes. The size of an image on disk is the sum of the sizes of its component layers. Docker allows you to commit changes to a running container, creating new image layers.

This lab will guide you through these concepts with hands-on practices, focusing on creating and modifying a Docker image with ubuntu as the base image. You will install and remove software within containers, observe the changes in image sizes, and understand the impact of Docker's Union File System (UFS) on image size.

Task: Building and Modifying Docker Images
Here, you will create a Docker image from the official Ubuntu image, install Git within a container, and commit the changes to create a new image. You will then modify this image by removing Git and observe how Docker manages image layers and size.

Steps:

Pull the ubuntu image from Docker Hub.

docker pull ubuntu

This command fetches the latest ubuntu image from Docker Hub and stores it in your local Docker repository.

Create a Container and Install Git:
Create a container from the ubuntu image and install Git.

docker run -d --name ubuntu-git-container ubuntu sleep infinity
docker exec -it ubuntu-git-container apt-get update
docker exec -it ubuntu-git-container apt-get install -y git

These commands run a container named ubuntu-git-container from the ubuntu image and install Git inside the container. The sleep infinity command keeps the container running. The apt-get update and apt-get install -y git commands update the package list and install Git, respectively.

Commit the Changes to Create a New Image:
Commit the container to create a new image with Git installed.

docker commit ubuntu-git-container ubuntu-git:1.0
docker tag ubuntu-git:1.0 ubuntu-git:latest

These commands commit the current state of the ubuntu-git-container container to a new image named ubuntu-git with a tag 1.0, and then tag this image as latest.

Check Image Sizes:
Check the sizes of all the images created.

docker images

Expected output:

Remove Git:
Create a new container from the ubuntu-git image and remove Git.

docker run --name ubuntu-git-remove --entrypoint /bin/bash ubuntu-git:latest -c "apt-get remove -y git"

This command runs a container named ubuntu-git-remove from the ubuntu-git:latest image with an entrypoint set to /bin/bash, and removes Git from the container.

Commit the Changes to Create a New Image with Git Removed:
Commit the container to create a new image with Git removed.

docker commit ubuntu-git-remove ubuntu-git:2.0
docker tag ubuntu-git:2.0 ubuntu-git:latest

These commands commit the current state of the ubuntu-git-remove container to a new image named ubuntu-git:removed and reassign the latest tag to this new image.

Check Image Sizes:
Check the sizes of all the images created.

docker images

Expected output:

Notice that even though you removed Git, the image actually same in size. Although you could examine the specific changes with docker diff, you should be quick to realize that the reason for the increase has to do with the union file system.

Remember, UFS will mark a file as deleted by actually adding a file to the top layer. The original file and any copies that existed in other layers will still be present in the image. When a file is deleted, a delete record is written to the top layer, which overshadows any versions of that file on lower layers.

It’s important to minimize image size for the sake of the people and systems that will be consuming your images. If you can avoid causing long download times and significant disk usage with smart image creation, then your consumers will benefit.