How to Update a Docker Image: A Step-by-Step Guide

Docker is a powerful containerization platform that allows developers to package applications and their dependencies into lightweight and portable containers. While managing Docker images, it is important to rebuild them efficiently to ensure consistency and smooth deployments. 

In this guide, we will explore how to update a docker image, understand the essential terminologies, and walk through the crucial steps. 

Updating a Docker Image

To update a Docker image, you need to follow these steps: 

Checking the Current Docker Image Version

First up, check the current image version running in your container:  

docker ps –format “table {{.ID}}\t{{.Image}}\t{{.Command}}\t{{.CreatedAt}}\t{{.Status}}”

To check the image version that is used to create a container: 

docker inspect –format='{{.Config.Image}}’ <container_id>

To list all downloaded images and their versions:

docker images

Pulling the Latest Version of a Docker Image

After you know what the latest version a Docker image is, you need to pull the image version of the Docker Hub using the command: 

docker pull <image_name>:latest

For a specific version:

docker pull <image_name>:<tag>

To verify the latest image is available:

docker images | grep <image_name>

Stopping and Removing the Existing Container

To update a Docker image, you first need to stop and remove the existing container. 

Find the running container:

docker ps

Stop the container:

docker stop <container_id>

Remove the container:

docker rm <container_id>

If the Docker container is set in volumes, you need to remove all the volumes. You can do so:

docker rm -v <container_id>cc

Updating the Docker Container with the New Image

Then, you can start a new container with the updated image: 

docker run -d –name <new_container_name> <image_name>:latest

If you need to use the same configuration as the previous container:

docker run -d –name <container_name> –restart unless-stopped -p <host_port>:<container_port> <image_name>:latest

Then verify:

docker ps

For containers managed by Docker Compose, update the service using:

docker-compose pull

docker-compose up -d

Check the container’s running image version:

docker inspect –format='{{.Config.Image}}’ <container_id>

Confirm the application is running correctly by checking logs:

docker logs <container_id>

If the container is using ports, test its availability:  

curl http://localhost:<port>

Cleaning Up Old Images to Free Space

You should regularly clean containers by removing old images to free up some space. You can do so by first listing all the current images: 

docker images

Then remove all the unused and dangling images by: 

docker image prune -f 

docker image prune -a -f

Or, you can remove a specific image by running:

docker rmi <image_id>

To free up additional space, remove stopped containers and unused volumes:

docker system prune -a

Automating Docker Image Updates

You can also automate the process to update a Docker image: 

1. Using Watchtower (for automatic updates)

You can Install and run Watchtower by:
docker run -d –name watchtower -v /var/run/docker.sock:/var/run/docker.sock containrrr/watchtower

To monitor and auto-update specific containers: 
docker run -d –name watchtower -v /var/run/docker.sock:/var/run/docker.sock containrrr/watchtower <container_name>

2. Using Docker Compose for Updates

If you are using Docker Compose yml to update images and restart services, you can use:  
docker-compose pull

docker-compose up -d

3. Using a Cron Job to Automate Updates

Schedule automated updates using Cron Job in Linux by:
 
crontab -e

0 0 * * * docker pull <image_name> && docker stop <container_name> && docker rm <container_name> && docker run -d –name <container_name> <image_name>:latest

Best Practices to Update a Docker Image

To avoid roadblocks when you update a Docker image, you can follow the following best practices: 

1. Use docker ps and docker inspect to verify the running image before you update a Docker image. 
2. Always pull a specific version (docker pull <image_name>:<tag>) to avoid unexpected updates.
3. Before you update production containers, test new images in a staging environment. 
4. Use docker-compose up -d or docker service update for seamless updates.
5. Clean up unnecessary images by running docker image prune -a.

Troubleshooting Common Update Issues

Issue	Possible Cause	Solution
Container doesn’t start after update	Configuration or environment variable changes	Compare old and new configurations with docker inspect
Image not updating	Local cache storing an old version	Use docker pull –no-cache <image> to force update
Port conflicts	Old container still running on the same port	Stop the old container with docker stop <container_id>
Data loss after update	Data stored inside the container, not in volumes	Use persistent storage (volumes or bind mounts)
Insufficient disk space	Too many old images accumulating	Run docker system prune -a to remove unused images
Service downtime	Updating without rolling restart	Use docker-compose up -d or docker service update
Permissions issues	Insufficient privileges to update/remove containers	Use sudo before Docker commands
Image not found error	Image name or tag incorrect	Verify name/tag with docker images or check the repository

Wrapping Up – Update a Docker Image

Following this tutorial will help you to update a Docker image easily and without any hiccups. You can also keep a track of the best practices and take help from the troubleshooting guide if you encounter any issues!