Managing Images

By the end of this exercise, you should be able to:

• Rename and retag an image
• Push and pull images from the public registry
• Delete image tags and image layers, and understand the difference between the two operations

Making an Account on Docker's Hosted Registry

1. If you don't have one already, head over to https://store.docker.com and make an account. This account is synchronized across two services:

   • Docker Store, for browsing official content
   • Docker Hub, for sharing community-generated content

   For the rest of this workshop, <Docker ID> refers to the username you chose for this account.

Tagging and Listing Images

1. Download the centos:7 image from Docker Store:

   [centos@node-0 ~]$ docker image pull centos:7
   

2. Make a new tag of this image:

   [centos@node-0 ~]$ docker image tag centos:7 my-centos:dev
   

   Note no new image has been created; my-centos:dev is just a pointer pointing to the same image as centos:7.

3. List your images:

   [centos@node-0 ~]$ docker image ls
   

   You should have centos:7 and my-centos:dev both listed, but they ought to have the same hash under image ID, since they're actually the same image.

Sharing Images on Docker Hub

1. Push your image to Docker Hub:

   [centos@node-0 ~]$ docker image push my-centos:dev
   

   You should get a denied: requested access to the resource is denied error.

2. Login by doing docker login, and try pushing again. The push fails again because we haven't namespaced our image correctly for distribution on Docker Hub; all images you want to share on Docker Hub must be named like <Docker ID>/<repo name>[:<optional tag>].

3. Retag your image to be namespaced properly, and push again:

   [centos@node-0 ~]$ docker image tag my-centos:dev <Docker ID>/my-centos:dev
   [centos@node-0 ~]$ docker image push <Docker ID>/my-centos:dev
   

4. Search Docker Hub for your new <Docker ID>/my-centos repo, and confirm that you can see the :dev tag therein.

5. Next, write a Dockerfile that uses <Docker ID>/my-centos:dev as its base image, and installs any application you like on top of that. Build the image, and simultaneously tag it as :1.0:

   [centos@node-0 ~]$ docker image build -t <Docker ID>/my-centos:1.0 .
   

6. Push your :1.0 tag to Docker Hub, and confirm you can see it in the appropriate repository.

7. Finally, list the images currently on your node with docker image ls. You should still have the version of your image that wasn't namespaced with your Docker Hub user name; delete this using docker image rm:

   [centos@node-0 ~]$ docker image rm my-centos:dev
   

   Only the tag gets deleted, not the actual image. The image layers are still referenced by another tag.

Conclusion

In this exercise, we praciced tagging images and exchanging them on the public registry. The namespacing rules for images on registries are mandatory: user-generated images to be exchanged on the public registry must be named like <Docker ID>/<repo name>[:<optional tag>]; official images in the Docker registry just have the repo name and tag.

Also note that as we saw when building images, image names and tags are just pointers; deleting an image with docker image rm just deletes that pointer if the corresponding image layers are still being referenced by another such pointer. Only when the last pointer is deleted are the image layers actually destroyed by docker image rm.