Creating Docker Tags

As you are iterating on your application, you will need to push new Docker images to the Docker registry. A natural question arises is how to version these images. This is where Docker tags come into the picture. Docker tags consist of three parts: the first part is namespace, the second part is image name, and the last part is where you specify the version.

By default, the Docker image name is made from slash-separated name components. Docker tags allow you to tag the images for clarification. In simple words, Docker tags include very useful information, such as OS version. This plays a very important role in the development lifecycle, as developers use these tags to differentiate the build version.

There are two types of tags: stable tags and unique tags.

Stable Tags

A stable tag is a tag that can be used and reused for maintaining the base image of your container. You should avoid stable tags for deployments because it continues to receive updates over time. There are two sets of stable tags: the major and the minor version:

• :1 – Specifies the major version.
• :1.0 – Also specifies stable tags for version 1.0
• :latest – Specifies the latest version of stable tags.

Unique Tags

It is recommended to use unique tags for deployments where you want to scale on multiple nodes. In simple terms, every image pushed to a Docker registry has a unique tag. You can use several patterns to generate unique tags, including Date-time stamp, Git commit, Manifest digest, and Build ID.

This tutorial shows you what Docker tags are and how to use them.

Requirements

• A server running Linux with Docker installed.
• A root password configured in your server.

Getting Started

Before starting, confirm that Docker is installed by running the following command:

You should get all the information about Docker from this command, as shown in the following output:

Docker Tag Syntax

The basic syntax of Docker tags is shown below:

Above, the term “image id” is the 12-character identification string of the image, and “tag” specifies the newly created versioning tag.

You can pull a specific version of an Ubuntu image from the Docker Hub registry using the image tag.

For example, to pull the Ubuntu 18.04 image from the Docker Hub, run the following command:

This command will search all images in the Docker Hub registry and download the image with the tag 18.04, as shown below:

Here, 18.04 is the tag of an Ubuntu image.

If you do not provide any tag, then the latest version of Ubuntu will be downloaded.

How to Tag an Image

First, list all available images in your Docker host system using the following command:

You should see the following output after entering the above command:

In the above output, you should see the Ubuntu image with the id 6526a1858e5d.

Tag the Ubuntu image with the name nginx using the following command:

Now, you can list your newly tagged image with the following command:

You should see your tagged image in the following output:

If you tag an image without specifying a tag, then it is given the latest tag by default.

Conclusion

As you have seen, Docker tags are very useful in the development lifecycle, allowing you to manage and maintain the version of your application. I hope you now have enough understanding of Docker tags.