A container is defined by its image as well as any configuration options you
provide to it when you create or start it. When a container is removed, any changes to
its state that aren’t stored in persistent storage disappear. You can create, start, stop,
move, or delete a container using the Docker API or CLI.

Constructing layers in an optimal manner can help reduce container size and improve performance. A typical Docker image consists of a base image layer and any additional image layers modifying the base. Once the user runs the docker run or docker create command, Docker adds a writable layer on top of the image and creates a container. The launch of Docker in 2013 jump started a revolution in application development – by democratizing software containers. Docker developed a Linux container technology – one that is portable, flexible and easy to deploy.

Difference between Docker Image VS Docker Container

Finally, as we saw in the interactive method, you can use the Docker images command to see the image you’ve just created. In the example above, we built the image from within the same directory as the Dockerfile and the context, as the . Argument simply tells the Docker daemon to build the image from the files and folders in the current working directory. Virtual machines (VMs) are an abstraction of physical hardware turning one server into many servers. Each VM includes a full copy of an operating system, the application, necessary binaries and libraries – taking up tens of GBs. Containers and virtual machines have similar resource isolation and allocation benefits, but function differently because containers virtualize the operating system instead of hardware.

For example, on Docker Hub, mysql is the name of the repository that contains different versions of the Docker image for the popular, open-source DBMS, MySQL. In simple terms, a base image is an empty first layer, which allows you to build your Docker images from scratch. Base images give you full control over the contents of images, but are generally intended for more advanced Docker users. The following table offers a summed-up list of differences between images and containers. The user can transfer the modifications recorded in the container layer to a new image using the docker commit command. During this procedure, Docker converts the container layer into an image layer and places it on top of the previous image layers.

Containers and Virtual Machines Together

Docker images can be created using either an interactive or Dockerfile method. Docker Hub is the default global marketplace for storing and distributing images. It has over 100,000 images created by developers that you can run locally. You can search for Docker Hub images and run them directly from Docker Desktop. CircleCI offers features such as Docker layer caching, which can significantly reduce build times by reusing the unchanged layers of your Docker images across builds. This is particularly useful when working with large images or when frequently updating images with small changes.

The application runs quickly and reliably from one computing environment to another. References to disk space in Docker images and containers can be confusing. Size refers to the disk space that the writable layer of a container uses, while the virtual size is the disk space used for the container and the writeable layer. The read-only layers of an image can be shared between any container started from the same image. A container image is composed of layers, added on to a parent image (also known as a base image). Layers make it possible to reuse components and configurations across images.

How to create a Docker image

A Docker daemon runs in the background to manage images, containers, and more. A client and the daemon communicate using sockets or through a RESTful API. One of the biggest benefits of containerization is that it helps developers to package their apps with all dependencies needed to run on any Linux distribution. Tools like Docker, Flatpak, and Snaps all have the same goal of making it easier to build, manage, and distribute containerized software. This is a JSON file that describes the image and provides metadata such as tags, a digital signature to verify the origin of the image, and documentation.

The method includes creating the Dockerfile and adding the commands needed for the image. For example, here is the Dockerfile of the MySQL image on Docker Hub, which can be used to create containers running the MySQL database. On top of this parent image, you can add layers that include additional software or specific configurations.

docker image ls

This is particularly important as Docker Official Images are
some of the most popular on Docker Hub. Typically, Docker Official images have
few or no packages containing CVEs. Next, you’ll need to know the name or ID of your running container instance. For more information about Docker images, read How to Create Docker Image from Dockerfile. Although it is part of the container, the image content remains read-only.

Another Docker client is Docker Compose,
that lets you work with applications consisting of a set of containers. It provides a viable, cost-effective alternative
to hypervisor-based virtual machines, so you why do we need docker can use more of your server
capacity to achieve your business goals. Docker is perfect for high density
environments and for small and medium deployments where you need to do more with
fewer resources.

When you use Docker, you are creating and using images, containers, networks,
volumes, plugins, and other objects. Docker Hub is a public
registry that anyone can use, and Docker looks for images on
Docker Hub by default. In addition to pulling a Docker image, you also learned about the layers of a Docker Image. In this hands-on, you will learn how to search and pull a container image using the Docker Desktop GUI. When using the –format option, the image command will either
output the data exactly as the template declares or, when using the
table directive, will include column headers as well. The reference filter shows only images whose reference matches
the specified pattern.

• Delve into their main differences to make well-informed business decisions.
• An image can be created from scratch or built on top of a previously existing image.
• For a Python web app, it’ll include the Python runtime, your app code, and all of its dependencies.
• Layers make it possible to reuse components and configurations across images.
• Alternatively, you can use an existing image to start a new container and make your changes in this new container.
• Docker streamlines the development lifecycle by allowing developers to work in
  standardized environments using local containers which provide your applications
  and services.

For a Python web app, it’ll include the Python runtime, your app code, and all of its dependencies. Building and deploying applications inside software containers eliminates “it works on my machine” problems when collaborating on code with fellow developers. You can manually build images using a Dockerfile, a text document containing all the commands to create a Docker image. You can also pull images from a central repository called a registry, or from repositories like Docker Hub using the command docker pull [name].

Fast, consistent delivery of your applications

The
Docker Official Images
are a curated set of Docker repositories hosted on Docker Hub. As COPY, but also able to handle remote URLs and unpack compressed files. Success in the Linux world drove a partnership with Microsoft that brought Docker containers and its functionality to Windows Server. Docker image is very light in weight so can be portable to different platforms very easily.