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When creating a Docker image, it is best practice to give it a descriptive and meaningful name. This process makes identifying and managing images more manageable, especially when dealing with many images.

This article discusses tagging images in Docker and Docker Compose.

Tagging images in Docker using Docker Build

Docker does not support tagging images in Dockerfile. Therefore, we use docker buildthe command to tag the image.

To name your Docker image, you can docker builduse the -t or --tag option when running the command.

Sample code:

$ docker build -t my-image .

In this command, the -t option specifies the name of the image as my-image. The name of the image must be unique within the registry where the image is stored.

If we try to build an image with an existing name, the build will fail with an error message.

When naming Docker images, it is best to use reverse domain name notation. This is because Docker automatically provides a hierarchical and unique namespace for images.

For example, if our company's domain name is example.com, you can name your Docker image com.example.my-image.

Tagging images in Docker Compose YAML files

We cannot tag images in Docker Compose like we do in vanilla Docker. However, the only difference is that we can tag images in the docker-compose.yml file.

Example of a YML file:

version: "3"
services:
    web:
        image: my-image:v3

Now, to build the image, we can use the same docker buildcommand and specify the name and version of the image using the -t or --tag option.

Sample code:

$ docker compose build -t my-image:v3 .

In this command, the -t option specifies the name and version of the image my-image:v3. The period at the end of the command .establishes the build context, which is the directory where we can locate the YAML file inside our system in Docker Compose.

When building an image, Docker Compose reads the YAML file and adds the specified name and version to the image using the image directive. We can then view a list of tagged images using the docker images command.

Sample code:

docker images

Output:

Copy code
REPOSITORY                  TAG                 IMAGE ID            CREATED             SIZE
my-image                     v3                 abc123              9 days ago          1GB

Overall, giving your Docker image a descriptive and meaningful name is critical for easy identification and management. Using reverse domain name notation and tags can help you create a hierarchical and unique namespace for your images and allow you to quickly identify different versions of an image.