Docker Image vs Docker Container

Docker Containers and Images are both used to package the application inside the container. However, there are huge differences in their work and composition. The infrastructure and execution of a Docker container completely depends on image composition.

What is Docker Image?

In Docker, images are considered as the building blocks that create the Docker container. Docker images are nothing but simple instructions that are used to dockerize the application inside the container. It contains the information about dependencies and libraries that are required to run the containerized application. Docker images are also referred to as blueprints, templates, or snapshots of Docker containers.

These images instruct the containers on how to build, manage, and deploy the user program or application inside the Docker container. Different official images are available that define the container’s main structure and the user can also generate the user-defined image to Dockerize the application through Dockerfile commands.

A Dockerfile is a simple instruction or command file that designs the image and specifies how to containerize the application. When a user executes the “docker build” command, the Docker Daemon will read the build context from the Dockerfile and generate the container blueprint(image) accordingly. This Dockerfile contains “FROM”, “RUN”, “CMD”, “ENTRYPOINT”, “ADD”, “COPY”, “EXPOSE” and many other similar commands to generate the Docker image.

What is a Docker Container?

The Docker container is another essential element of the Docker working environment. These containers dockerize the applications, programs, and software. When the user builds the application, the user thinks about installation medium, OS version, and many other compatibility features. However, while building the application in a containerized environment, the user does not need to think about these complex configurations. 

The Docker containers build, deploy, and ship the application by encapsulating all its dependencies, source code, and essential configurations. These containers can be easily installed and executed on any system that has Docker setup to deploy a containerized application. This will also reduce the chances of dependency error occurrence that usually occurs while installing an application on any other system.

Docker Image Vs Docker Container. What are the Key Differences?

Docker images and containers are closely related to each other and work together to Dockerize the user application. By reading the above sections, now you are able to understand how these are tightly coupled with each other. However, there are huge differences in their composition.

The images are blueprints of containers that are utilized to generate the container to dockerize the application. But an image can exist without a container. In contrast, containers always need images in their composition. Docker images are created through Dockerfile, but Docker containers are created through images.

Another major difference between these two is that the Docker image file consists of image layers to keep the image size smaller, and these layers are only readable layers. In contrast, the container contains layers of an image as they are generated from a Docker image. However, containers have an extra read/write layer on which users can write the new updates.

Containers can be modified and updated as per requirements. However, images are immutable which means these cannot be updated. To update the image, every time Docker users need to recreate the image from scratch.

Here are some key differences between Docker image and Docker container in tabular form:

Images Commands 

Here are some commands that are used to generate and manage Docker images:

Containers Commands

The commonly used commands to manage or operate the containers are listed below in tabular form:

How Containers and Images Work with Each other

Docker containers are completely dependent on Docker images. To dockerize the application, the user first needs to have the Docker image containing the instructions on how to generate the container and how to dockerize the application inside the container. 

In the Docker runtime environment, first, generate the image from a simple file instruction named Dockerfile. Then, the generated blueprint (image) is used to create and execute the Docker container. To practically view the working of the Docker container and image, follow the example given below.

Step 1: Dockerfile Instructions

To create a new image from the Dockerfile commands, first, make a file in any text editor and add the below snippet:

FROM python
WORKDIR /src/app
CMD python3 -c "print('Welcome to Hands-on.Cloud Docker Tutorial')"

For demonstration, we have specified the instruction to dockerize the simple Python program.

Step 2: Generate an Image

To generate the blueprint (image) for the container, execute the “docker build -t <image-name> .” command. Here, “.” is utilized to read the Dockerfile build context from the currently opened directory:

docker build -t py .

Step 3: Create Docker Container

Now, from the generated images, create and fire up a container. To do so, simply execute the “docker run <image>” command:

docker run py

That is all about Docker images and containers and the key differences between them.

Conclusion

Docker images and containers are interconnected with each other but there is a difference in their composition and properties such as Docker images are created through simple Dockerfile commands. In contrast, containers are created through images. Docker images are read-only but the container contains a single writable layer as a top layer. Images cannot be modified but containers can be upgraded and modified. Images are logical entities, but containers are real-time applications. This write-up has explained the key difference between docker images and containers.