Docker Pull – The Complete Guide

By offering a portable and effective containerization platform, Docker has transformed the world of software development and deployment. Among Docker’s several effective capabilities, pull stands out as an essential action that enables users to get container images from distant sources. In this blog, we’ll explore Docker Pull’s nuances and examine its advantages and scenarios.

Kindly go through our explanation video in order to get a better understanding about Docker!

Introduction to Docker Pull

Docker is a popular open-source platform that allows developers to automate the deployment and scaling of applications using containerization. Containerization provides a lightweight and isolated environment to run applications consistently across different systems. In the Docker ecosystem, the process of acquiring container images from a remote repository is known as “pulling.”

The Docker pull command plays a crucial role in obtaining container images from a specified Docker registry. It allows developers to fetch pre-built images that can be used as the base for building and running containerized applications. By pulling images from a registry, developers can save time and effort by leveraging existing solutions and dependencies instead of starting from scratch.

How to run a Docker Container:

To use the ‘docker run’ command, you must specify the image’s name to be executed. Additionally, the command that will be run within the container can be given.

To execute the echo “Hello, world!” command inside the container, the following command gives you to run the most recent version of the Ubuntu image:

docker run ubuntu echo "Hello, world!"

The docker run command initiates a new container and executes the specified command within it. The container will continue running until the command completes.

How to Use the Docker Pull Command

When the Docker pull command is executed, several steps are involved in the process of acquiring the requested container image. Some of them are defined further:

• Identification of the Image – The first step is to identify the specific image to be pulled. This is done by specifying the image name along with its repository and, optionally, a tag that represents a specific version or variant of the image. For example, to pull the official NGINX image from Docker Hub, the command would be docker pull nginx.
• Contacting the Docker Registry – Once the image name is identified, the Docker daemon contacts the Docker registry specified in the image name (e.g., Docker Hub). The registry is a centralised repository that stores and distributes Docker images. By default, Docker Hub is the public registry, but private registries can also be utilised.
• Authenticating and Authorising – If the registry requires authentication and the user has not previously logged in, the Docker daemon prompts the user to provide valid credentials. The authentication process ensures that only authorised users can access private or restricted images.
• Requesting the Image Manifest – After the authentication process, the Docker daemon sends a request to the registry for the image manifest. The manifest contains metadata about the image, such as the layers that compose it, its dependencies, and other configuration details. The manifest allows Docker to understand how to assemble and run the container based on the pulled image. This metadata also includes runtime configurations defined within the image, such as HEALTHCHECK instructions. For example, Docker Engine 25.0 introduced new health check options that are part of this downloaded metadata and may affect image pulling by altering how a container’s health is monitored at runtime in specific scenarios.
• Downloading Image Layers – Once the manifest is received, the Docker daemon starts downloading the image layers one by one. Docker uses a layered architecture, where each layer represents a specific part of the image’s filesystem. Layers are stacked on top of each other, allowing for efficient storage and sharing of common layers across multiple images. Docker downloads and verifies each layer’s integrity before moving on to the next.
• Assembling the Image – As the layers are downloaded, Docker progressively assembles them into a complete image based on the instructions provided in the manifest. Each layer contributes to the final filesystem state and configuration of the container. The layering approach allows for quick and efficient updates since only the modified or new layers need to be downloaded and applied.
• Image Caching – To optimise the pulling process, Docker caches downloaded layers locally on the host machine. When subsequent pulls of the same image or images with shared layers are performed, Docker can utilise the cached layers, significantly reducing download times. The cache is periodically updated and can be managed using Docker’s caching mechanisms.
• Recent Docker Engine Features – In addition to the core process, recent versions of Docker Engine include features that refine the pull behaviour. Modern engines, for instance, have improved support for multi-platform images. When pulling an image, the engine can automatically detect the host machine’s architecture (e.g., ARM or x86) and pull the correct image variant, streamlining cross-platform development and deployment.

Example

Let’s walk through an example to illustrate how the Docker pull command works. Suppose we want to pull the latest version of the Ubuntu image from Docker Hub:

docker pull ubuntu

• The Docker daemon receives the docker pull ubuntu command.
• It identifies the image as “ubuntu” without specifying a tag, which defaults to the “latest” tag.
• The Docker daemon contacts the Docker Hub registry.
• If authentication is required, the user provides valid credentials.
• The Docker daemon requests the image manifest for the “ubuntu:latest” image.
• The Docker Hub registry responds with the manifest, which contains information about the layers and configuration of the “ubuntu” image.
• The Docker daemon starts downloading the image layers one by one, verifying their integrity.
• As the layers are downloaded, Docker assembles them into a complete filesystem, representing the “ubuntu” image.
• Once the image is fully assembled, it is ready for use as a base for running containers.
• If the “ubuntu” image has shared layers with other images previously pulled, Docker utilises the cached layers, reducing download time.
• The pulled “ubuntu” image is now available on the local machine and can be used to create and run containers.

By understanding the inner workings of the Docker pull command, developers can efficiently acquire the necessary container images, reducing development time and ensuring consistency across environments.

Scenario

In this section, we will explore various scenarios related to pulling Docker images. Each scenario presents a specific challenge or requirement, and we will provide step-by-step explanations to overcome them.

Scenario 1 – Pulling the Image from a Different Registry or Private Registry

To implement a Docker image pull from a registry other than the default Docker Hub or from a private registry, follow these steps in some instances.

1. Identify the Registry: Determine the URL or address of the registry where the desired Docker image is stored. This could be a private registry hosted internally or a different public registry like gcr.io (Google Container Registry).
2. Authenticate with the Registry: If the registry requires authentication, ensure you have the necessary credentials. Use the docker login command to authenticate. For example, docker login registry.example.com.
3. Pull the Image: Once authenticated, use the docker pull command with the full image name, including the registry URL. For example, docker pull registry.example.com/myimage:latest.

By following these steps, you can successfully pull an image from a different or private registry.

Scenario 2 – Pulling a Repository with All Available Tags

Sometimes, you may need to retrieve an entire repository with all its available tags. Note: The docker search command does not list tags. You would typically use the registry’s API or a separate script for this.

1. Pull the Images: Within the loop, use the docker pull command along with the repository name and the current tag to pull each image. For example, docker pull ubuntu:$tag.
2. List the Repository’s Tags: You can use a command-line tool like curl combined with jq to query the Docker Hub API for tags. For example, to list tags for the ubuntu image: curl -s 'https://registry.hub.docker.com/v2/repositories/library/ubuntu/tags/' | jq '."results"[]["name"]'
3. Loop Through the Tags: Write a script (e.g., in Bash) to iterate over the list of tags obtained in the previous step.

Scenario 3 – Pulling Docker Images that are not Signed with Content-Trust Enabled

Content trust ensures the integrity and authenticity of pulled images by verifying signatures. However, there may be situations where you need to pull images that are not signed.

• Re-Enable Content Trust: After successfully pulling the unsigned image, you can re-enable content trust by setting the DOCKER_CONTENT_TRUST environment variable back to 1..
• Disable Content Trust Temporarily: Temporarily disable content trust by setting the DOCKER_CONTENT_TRUST environment variable to 0.
  • On Linux/macOS: export DOCKER_CONTENT_TRUST=0
  • On Windows (PowerShell): $env:DOCKER_CONTENT_TRUST="0"
• Pull the Image: Use the docker pull command followed by the image name and tag to pull the unsigned image. For example, docker pull myimage:latest.

Scenario 4 – Pulling the Image without Verbose Output

To pull an image with a more concise output, use the --quiet or -q option.

• Pull the Image with Suppressed Output: Execute the docker pull command with the --quiet flag. This will only output the final image ID upon success. docker pull --quiet nginx:latest

Scenario 5 – Handling Common Pull Errors

When pulling images, you may encounter errors. Understanding them is key to troubleshooting.

• Error Example: manifest for my-image:nonexistent-tag not found
  • Meaning: The specific tag you requested does not exist in the repository.
  • Solution: Check the image repository on Docker Hub (or your private registry) to find the correct, available tags for the image.
• Error Example: pull access denied for my-private-image, repository does not exist or may require 'docker login'
  • Meaning: You are not authorisedenticated to access a private repository, or the image name is incorrect.
  • Solution: Run docker login <your-registry-url> with your credentials before attempting the pull again.

Docker Pull: Advantages

Docker pull is a fundamental command in Docker that allows users to download container images from Docker Hub or other container registries. This section will explore the advantages of using Docker Pull and how it can greatly benefit developers and operations teams in their containerised environments.

• Efficient Image Distribution – One of the significant advantages of docker pull is its efficient image distribution. Docker Hub acts as a centralised repository, making it easy to share images. When pulling an image, Docker only downloads the layers that are missing locally, optimising bandwidth.
• Version Control and Image Tagging – docker pull enables version control through image tagging. Each image can have unique tags (e.g., 1.2.3, latest), allowing developers to pull specific, consistent versions for development, testing, and production.
• Enhanced Security with Docker Content Trust – Docker Content Trust provides a critical security advantage by ensuring image authenticity. For example, by enabling content trust (export DOCKER_CONTENT_TRUST=1), a developer pulling a critical database image like postgres:16 can be certain they are getting the official, unmodified image signed by the publisher. If the image signature is invalid or missing, the pull will automatically fail, preventing the use of a potentially compromised image.
• Rapid Deployment and Scalability – Container images can be quickly downloaded and deployed, enabling rapid deployment and scalability. Instead of manually configuring environments, developers can simply pull the required images and run containers, significantly speeding up the development-to-production lifecycle.
• Simplified Collaboration and Dependency Management – docker pull simplifies collaboration. By pulling the same container image (e.g., node:20-alpine), team members can ensure they are working with identical environments and dependencies, eliminating “it works on my machine” issues.
• Offline Availability and Portability – docker pull allows users to download images and store them locally. Once an image is pulled, it can be saved to a tarball (docker save) and loaded onto an offline or air-gapped system (docker load), enabling seamless collaboration and deployment without requiring a live internet connection.

Conclusion

Docker pull is an essential command for Docker users. It allows you to easily download pre-built Docker images from Docker registries, saving you time and resources. By following the step-by-step instructions in this guide, you can learn how to use the Docker pull command to pull Docker images like a pro. So whether you’re new to Docker or an experienced user, start using the Docker pull command today and experience the benefits of this powerful tool.