Docker Images vs Containers – All You Need To Know

Docker Tutorial - Images vs Containers

Docker Images vs Containers? You might be confused about both terms as I was back in 2017. Before we dive deep into and start using Docker more practically, you need to develop understanding about both terms images and containers.

Docker image vs container can be a bit complicated and challenging to understand, but you landed on the right page here as I will break them down and make them easy to understand.

The simple answer to the images vs containers comparison is:

Docker images are nothing more than a template and Docker containers are the running instances created from those templates.

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Docker is a platform for developers to build, test and run applications with containers aka containerization. Docker simplifies the distribution of your application across multiple platforms. By understanding the process of how Docker use and store data through images and containers, will help you to design future proof Docker applications.

Using applications in containers will lead to several advantages such as:

  • Docker containers are portable, you can build your application locally and then deploy them in different environment.
  • Containers are lightweight as they only carry the shared host.
  • Containers are stackable.

What are Docker Images?

In simple words, a Docker image is a file, comprised of multiple layers, which we use to execute code in a Docker container.

If you have ever used the virtual machines, you might be familiar with the concept of images. An image is a snapshot of the Docker virtual machine at a specific time. Docker images are different from other virtual machine images, as an image does not have state and it always remains the same (never changes). Once we have created an image, we can delete it but we can not modify it.

Docker Image
Docker Image

Docker images contain the necessary software and configurations for applications. We can build a distributable image that contains pre-configured software like Nginx, NodeJS, Redis and so on.

Docker Images Key Points:

  • A Docker image a collection of files with some metadata and these files form a Docker container.
  • Each image is based on multiple layers, stacked on top of each other.
  • Images can share layers to optimize the disk usage and transfer time.
  • An image is a ready only template which we use to create a container.
  • You can build your own image or you can use developed by others.
  • Docker images are stored on the Docker Hub or local Registry.

What is DockerFile?

Docker images are created from a Dockerfile with the docker build command. Dockerfile is a set of instructions that build the Docker image.

Dockerfile
Dockerfile

When we run the Docker run command and specify WordPress, Docker uses this file to build the image of a WordPress.

What are the Docker Layers?

Each instruction in a Dockerfile result in a new image layer, which can also be used to start a container. The new layer is created by starting a container of the previous layer. When a DockerFile instruction completed successfully and the final image will be created and all intermediate containers will be deleted.

Docker Image Repository

Once you have created a Docker image using a Dockerfile, you can host this image locally or make it public on the Docker Hub repository. You can send this image to a friend and he/she can start a new virtual machine based on your image. Their virtual machine will run the same as yours.

Developers around the world share their images with other developers through Docker Hub. There is an image available for almost any software in the world.

What are Docker Containers?

A Docker container is a runnable instance of the image. Using the Docker CLI (Command Line Interface), we can create, start, stop or delete a container. Docker containers can connect to other networks and we can also attach the storage to them. Using the Docker run command, we can create containers form images.

Docker Container Key Points:

  • Docker containers are isolated application platforms.
  • They contains everything which we need to run an application.
  • We can identify the containers using their IDs.
  • By removing the containers, we can clean up the host.

Docker Image vs Container

Until now, you will have a clear understanding of what is a Docker image and what is a container. Images and Containers are totally different from each other but they work together. Each image provides a reproducible and shareable virtual environment across the office or world. Containers are built on those images to run the application in different environments.

Here are the key differences:

  1. An image is a read-only filesystem, while a container can encapsulate set of processes which are running in the read-write copy of that filesystem.
  2. Fraom a given image, we can start a docker container.
  3. Instead of regular copy, copy-on-write use to optimize container boot time.
  4. The top writable layer is a major difference. When we start a container all-new addition of data or modification of existing data to the container, stores in this top writable layer. When we delete a container, the top writeable layer is also erased. But the underlying image, the container is running on the basis of, remains unchanged.

Read More

To learn more about Docker platform, please checkout our post about What is Docker? A Brief Introduction.

Wrap Up

A common way to explain the images and containers is to think of a an image as a class and a container is an instance of that class. Docker reduces the development time and enables you to speed up testing processes by setting up the new identical systems.

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