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 microsoft/nanoserver image from Docker Store:

   PS: node-0 Administrator> docker image pull microsoft/nanoserver
   

2. Make a new tag of this image:

   PS: node-0 Administrator> docker image tag microsoft/nanoserver mynanoserver:dev
   

   Note no new image has been created; mynanoserver:dev is just a pointer pointing to the same image as microsoft/nanoserver.

3. List your images:

   PS: node-0 Administrator> docker image ls
   
   ...
   mynanoserver                  dev                   4c872414bf9d        3 weeks ago         1.17GB
   microsoft/nanoserver          latest                4c872414bf9d        3 weeks ago         1.17GB
   ...
   

   You should have microsoft/nanoserver and mynanoserver:dev both listed, but they ought to have the same hash under image ID, since they're actually the same image. (Note you'll have a lot of other images, too - these were pre-downloaded for this workshop. On your own machines, you'll have to download the images you want using docker image pull like above).

Sharing Images on Docker Hub

1. Push your image to Docker Hub:

   PS: node-0 Administrator> docker image push mynanoserver:dev
   

   You should get an 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:

   PS: node-0 Administrator> $user="<Docker ID>"
   PS: node-0 Administrator> docker image tag mynanoserver:dev $user/mynanoserver:dev
   PS: node-0 Administrator> docker image push $user/mynanoserver:dev
   

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

5. Next, write a Dockerfile that uses $user/mynanoserver: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:

   PS: node-0 Administrator> docker image build -t $user/mynanoserver: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:

   PS: node-0 Administrator> docker image rm mynanoserver:dev
   

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

Conclusion

In this exercise, we practiced 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 on Hub 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.