Tagging

we can get the list of images by,

docker image ls

In the image list, we can notice docker images does not have a name.

Only official images can have root namespace. Other images repository name be like user_name/repository_name

The image tag is not exactly a version or branch, it's the git tag and it can represent both.

In docker world, latest tag is more of a meaning to default or stable image. latest tag in docker image does not imply the latest image. It is possible to take older image and tagged as latest

Let's assume, we have the nginx image with latest tag. If not get it,

docker pull nginx:latest

The latest nginx also tagged with mainline. So If we try to pull the nginx:mainline,

docker pull nginx:mainline

Instead of download, it will use the downloaded nginx:latest image layer.

Now, if we print the list of images,

docker image ls

We will see the nginx image has same IMAGE ID but different tag.

Tag Existing Image

We can create a tagged target image from the existing image by tag command,

docker image tag nginx bmshamsnahid/nginx

If we now see the image list,

docker image ls

We will see a new nginx image repository bmshamsnahid/nginx with latest image. This new image should also have the same image id.

If we do not specify the tag, it will take the latest tag by default

Pushing Image to Docker Hub

Make sure you have a account in docker hub and you are logged in to your local machine,

docker login

Now, push our latest image to docker hub,

docker push bmshamsnahid/nginx

In docker hub, we should see the image bmshamsnahid/nginx with latest tag.

Now to add another tag, testing, we can run,

docker tag bmshamsnahid/nginx bmshamsnahid/nginx:testing

We can verify the testing tag in local image list by,

docker image ls

In the output list, we should see a image bmshamsnahid/nginx with testing tag.

We can push this newly testing tagged image to docker hub by,

docker push bmshamsnahid/nginx:testing

Since the image layer is same, only the tag is different, we should see Layer already exists in the console. Also it should add a image tag testing in the docker hub.

So if an image layer exist in the docker hub, it does not upload twice. And same for the local machine, if an image layer exists in the local machine cache, it does not download twice.

To upload a image in private repository, we first have to create a private repository in the docker hub