Docker - Using Image Tags

Image tags are used to describe an image using simple labels and aliases. Tags can be the version of the project, features of the Image, or simply your name, pretty much anything that can describe the Image. It helps you manage the project's version and lets you keep track of the overall development process. Docker tags are labels for container images, used to differentiate versions and variants of an image during development and deployment. Docker tags will help you identify the various versions of docker images and help distinguish between them. Docker image will help us to build continuous deployment very quickly.

What Is Docker Image Tag?

A Docker image tag is simply a label that helps you identify a specific version of a Docker image. It’s used to keep track of different image versions, so you can easily pick which one to run or deploy. You’ll see the tag added to the image name after a colon, like this:

<image_name>:<tag>

Example

nginx:latest

The 'nginx:latest' tag means you're using the newest version of the Nginx Docker image available. Here’s what each part means:

• nginx : This is the actual Docker image name, and it contains everything you need to run Nginx, a widely used web server and reverse proxy.
• :latest : This tag indicates you want the most recent stable version of the image.

When you use 'nginx:latest', Docker will pull the latest stable version from Docker Hub (or wherever the image is hosted). It’s a common choice in development when you want the newest updates. In production, however, you’d usually specify a specific version, like 'nginx:1.21', to avoid unexpected changes from future updates.

Why Tag A Docker Image?

When working on personal projects with Docker images, you might not think much about tagging. But in a real-world team setting, where multiple Docker images are in circulation, having a clear and consistent tagging strategy becomes essential.

Here’s why tagging Docker images is so beneficial:

• Version Control: Tags let you manage different versions of the same application, which makes it easier to roll back if something goes wrong. This is especially helpful if a new release has a bug or causes compatibility issues.
• Traceability and Accountability: Tags help track the origin and changes in an image, offering insights into how an application has evolved. This can make it easier to trace when and by whom specific updates were made.
• Streamlined Deployment and Automation: A structured tagging strategy supports automated deployment processes, helping manage the application lifecycle. For instance, CI/CD pipelines can use tags to automatically build, test, and deploy new application versions.
• Simpler Troubleshooting: With a solid tagging approach, finding the correct version during troubleshooting is quicker. This reduces the time spent diagnosing and resolving issues.

How To Tag A Docker Image?

We can tag a Docker Image by the following methods:

Tag a Docker Image During the Build Process

• You can specify a tag to the image right when you are building it using the -t flag. If you don't specify a tag, it is automatically tagged with the latest tag.

sudo docker build -t <image-name>:<tag-name>

• You can also specify the Image tag you want to pull in the Dockerfile. Let's say you have a Dockerfile to pull an Ubuntu Image with the latest version.

FROM ubuntu:latest

• If you want to build the Image with a custom tag called my-ubuntu, you can use the following command.

sudo docker build -t tag-demo:my-ubuntu .

Tag a Docker Image After the Build Process

• Docker images can be tagged by using Docker CLI as shown below.

docker tag Source_Image[:Tag] Target_Image[:Tag]

• In the place of SOURCE_IMAGE mention the image that you want to tag and TARGET_IMAGE with the desired name for the tagged image. To differentiate versions, you can also optionally specify a TAG for both the source and target images.

Example

• If you have an image named 'gfg_app' and you want to tag it as version 2, you can use the command:

docker tag gfg_app gfg_app:2

• This will create a new image with the tag '2' based on the original 'gfg_app' image.

Note: Ensure you have the necessary permissions and access to the Docker daemon to perform tagging operations.

Build An Image With A Single Docker Tag

• While building the image by using Dockerfile also you can tag the image by using the following command.

docker build -t gfg/gfg_app: 1 .

• "-t" represents the option called tag while 'gfg' represents the name of the repository and 'gfg_app' represents the name of the image with tag1.

Build An Image With Multiple Docker Tags

• Docker can assign multiple tags to a single image. This can be done while building the image as shown below.

docker build -t gfg/gfg_app: 1 -t gfg/gfg_webapp: 1

• Here we are using the '-t' command two times for tagging the image with two different names.

Build An Image Without Any Tag

• Building the image with only a tag is always not an option. You can build the image without using a tag also the main purpose of the tag is to keep the progress of the images you are building.

docker build -t gfg/gfg_app .

Tag An Image Using Docker Tag Command

• You can also tag an Image directly using the tag sub-command.

sudo docker tag <imageId> <imageName>/<tagName>

• You can see that the new tag has been assigned to the Image.

Tagging A Docker Image For Docker Hub

• To push an image to Docker Hub, you need to tag it with your Docker Hub username. Here’s the command:

docker tag <imageId> <DockerHubUsername>/<ImageName>:<tagName>

Example

docker tag 40e3ce7f18ef deeksha123456789/gfg:latest

Use of Tag In Docker Pull Command

• You can pull a Docker Image using the pull sub-command. You can specify the tag of the Image that you want to pull. Note that if you don't specify a tag, it will automatically pull the latest version of the Image by appending the 'latest' tag.

sudo docker pull alpine:3.6

Best Practices For Docker Image Tagging

Use Version Numbers

• Tag your images with specific versions like '1.0', '1.1.2' to clearly show updates.

Apply Multiple Tags for Flexibility

• Tag the same image with both '1.0' and '1.0.3' so you can reference it by major or exact version.

Avoid 'latest' in Production

• Instead of relying on 'latest', use specific tags like '2.1' or 'stable' to avoid unexpected changes.

Label for Different Environments

• Tag images for their environment, like '2.0-prod' or '2.0-dev', to make it clear where each is intended to run.

Automate Tags with CI/CD

• Set up CI/CD to automatically tag images with build numbers or commit hashes to keep everything consistent.

Troubleshooting Of Docker Issues Using Image Tags

The following are the some of the troubleshooting of Docker Issues using Image Tags:

• Specify Correct Tags: Use specific version tags to avoid inconsistencies.
• Identify and Roll Back: Roll back to stable versions if issues occur.
• Track Changes: Use descriptive tags for easier issue identification.
• Avoid Tag Conflicts: Ensure unique tags for custom images to prevent conflicts.

Conclusion

In conclusion, Docker image tagging is essential for managing and deploying containerized applications effectively. Tags allow developers to track different image versions, facilitating a smoother workflow in both development and production environments. By implementing a clear tagging strategy, teams can enhance version control, traceability, and automation in their deployment processes.