"naming to docker.io", explanation and prevention?

Question

Naming to docker.io/library/imageName

When I build an image from a dockerfile, I see this statement appear in the docker build log, as the last statement of the build log that prints in the console.

What does this mean? Does this mean a copy of the image has been pushed to docker.io?

If so, is there any way to prevent this? It seems to continue happening even if I run docker logout.

If it matters, I am currently using a fresh install of docker for windows with wsl2 integration, and am running the docker build command within ubuntu linux.

Kimsey

Can you add more context in the output around this message? (Lines before, lines after, the build command you use) – Ymir

Answer 1

docker.io/library is the default registry applied when you don't specify a registry URL.

When you specify -t in the build command, the text before the / is considered the registry URL.

Here is an example of building an image, one without a specific registry and one with:

# no registry specified
➜  docker build . -t my-image
[+] Building 0.5s (6/6) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                                                                                                                                   0.0s
 => => transferring dockerfile: 86B                                                                                                                                                                                                                                                                                                                                    0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/library/ubuntu:20.04                                                                                                                                                                                                                                                                                                        0.0s
 => CACHED [1/2] FROM docker.io/library/ubuntu:20.04                                                                                                                                                                                                                                                                                                                   0.0s
 => [2/2] RUN cat /etc/lsb-release                                                                                                                                                                                                                                                                                                                                     0.4s
 => exporting to image                                                                                                                                                                                                                                                                                                                                                 0.0s
 => => exporting layers                                                                                                                                                                                                                                                                                                                                                0.0s
 => => writing image sha256:4a97ceefb5314bdf91886a28142c9b0b33c992c94b1847d5ae1b38723b2279e3                                                                                                                                                                                                                                                                           0.0s
 => => naming to docker.io/library/my-image                                                                                                                                                                                                                                                                                                                0.0s

# registry set to "my.docker.registry"
➜ docker build . -t my.docker.registry/my-image
[+] Building 0.1s (6/6) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                                                                                                                                   0.0s
 => => transferring dockerfile: 36B                                                                                                                                                                                                                                                                                                                                    0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/library/ubuntu:20.04                                                                                                                                                                                                                                                                                                        0.0s
 => [1/2] FROM docker.io/library/ubuntu:20.04                                                                                                                                                                                                                                                                                                                          0.0s
 => CACHED [2/2] RUN cat /etc/lsb-release                                                                                                                                                                                                                                                                                                                              0.0s
 => exporting to image                                                                                                                                                                                                                                                                                                                                                 0.0s
 => => exporting layers                                                                                                                                                                                                                                                                                                                                                0.0s
 => => writing image sha256:4a97ceefb5314bdf91886a28142c9b0b33c992c94b1847d5ae1b38723b2279e3                                                                                                                                                                                                                                                                           0.0s
 => => naming to my.docker.registry/my-image

The image tag specifies where the image will be uploaded if you do a docker image push <image name>.

In the first case, if you did docker image push my-image it would push to docker.io/library.

In the second case, if you did docker image push my.docker.registry/my-image it would push to a registry at the URL my.docker.registry (assuming it exists)

Bowker

so assuming I don't push, nothing actually goes to the registry? It is just giving me a notification message that if I push, that is the registry it will go to? @Bowker – Kimsey

Correct. It is just stored locally unless you run "docker image push <tag>" – Bowker

and can you actually push to a different registry to override to default? – Thirtyone

@Thirtyone in order to that you have to rename the image tag using docker tag <current_image_name> <new_image_name>, so for example: docker tag docker.io/library/my-image my.docker.registry/my-image – Himalayas