Pulling and Pushing Images to/from Registries in Docker

You can get training on our this article, which delves into the crucial processes of pulling and pushing Docker images to and from registries. For intermediate and professional developers, understanding these operations is essential for efficient container management and deployment practices. In this article, we will explore the intricate details of Docker image registries, focusing on step-by-step guidance and practical examples using a Python-based application.

Before we dive into the specifics of pushing and pulling images, let's clarify what a Docker registry is. A Docker registry is a storage and distribution system for named Docker images. By using registries, developers can share their applications with others, enabling easy deployment across various environments. The most popular Docker registry is Docker Hub, but there are alternatives like Amazon ECR, Google Container Registry, and private registries.

How to Pull an Image from a Registry

Pulling an image from a Docker registry is equally straightforward. This process allows you to download an existing Docker image to your local machine for use. Here’s how to pull the image we just pushed.

Pull the Docker Image

To pull the image from the Docker registry, you would use the following command:

docker pull myusername/my-flask-app:latest

This command fetches the latest version of your application image from Docker Hub.

Run the Docker Image

Once the image is pulled, you can run it using the following command:

docker run -d -p 80:80 myusername/my-flask-app:latest

This command runs the container in detached mode (-d) and maps port 80 of the container to port 80 on your host machine, allowing you to access your Flask application via a web browser.

Example of Pulling with Docker Compose

If you have defined your services in a docker-compose.yml file, you can easily pull all images defined in the file with:

docker compose pull

This is particularly useful when working in a team environment where multiple developers might need to synchronize their local environments.

How to Push an Image to a Registry

Pushing an image to a Docker registry involves several steps. First, you must build your Docker image and tag it appropriately. Once tagged, you can authenticate yourself with the registry and then push the image. Here’s a step-by-step guide using our previously defined Python Flask application.

Build the Docker Image

Assuming you have a directory structure similar to this:

/my-flask-app
│
├── app.py
├── Dockerfile
└── requirements.txt

You can build your Docker image using the following command:

docker build -t myusername/my-flask-app:latest .

In this command:

• myusername/my-flask-app is the name of the image.
• latest is the tag that signifies the version of the image.

Log In to the Docker Registry

Before pushing your image, you need to log in to Docker Hub (or any other registry you are using). Execute the following command and provide your Docker Hub credentials when prompted:

docker login

Push the Docker Image

Now that you are logged in, you can push your image to the Docker registry with the following command:

docker push myusername/my-flask-app:latest

This command uploads your image to Docker Hub, making it accessible for future deployments.

Example of Pushing with Docker Compose

If you're using Docker Compose for your applications, you can define the image in your docker-compose.yml file:

services:
  web:
    build: .
    image: myusername/my-flask-app:latest

You can then push the image using:

docker compose push

This streamlines the process, especially when working with multiple services.

Summary

We explored the processes of pushing and pulling Docker images to and from registries, specifically using a Python Flask application as an example. We covered the essential steps, from building and tagging images to logging in and executing push/pull commands.

Understanding these processes enhances your capability to manage Docker images efficiently, facilitating a smoother development workflow and deployment strategy. As you advance in Docker usage, consider integrating practices such as automated builds and CI/CD pipelines to streamline your image management further.

Last Update: 20 Jan, 2025