Docker says "...: Already exists" even though image has been removed beforehand

Question

Just in the process of learning Docker. Tried to run the hello-world example:

# docker run hello-world

Worked.

Removed the image using

# docker image rm -f <image-id>

Worked.

# docker images

Shows no images.

Now, if I try to run the hello-world example again, it shows this:

It mentions:

2db29710123e: Already exists

Does this mean, the image already exists on the local host machine, or the image does exist on the Docker Registry, and this is a new pull?

Any insight would be much appreciated.

Answer 1

When Docker says that 2db29710123e: Already exists, yes, it already exists on your local machine. It doesn't exist as part of the hello-world image though, because you force-removed that beforehand. However, why did you have to force-remove it in the first place? Why wasn't a regular remove enough? Here is the message it shows, when running docker image rm hello-world:

Error response from daemon: conflict: unable to remove repository reference "hello-world" (must force) - container 4d198a310ed5 is using its referenced image feb5d9fea6a5

This means you still have a container (from the previous hello-world test) which references that image. So when you force-remove, the image (tag or label) is removed from docker images, but the underlying data is still there, locally, because of the existing container.

However, if you remove the container (with docker container rm or docker container prune for example) before running the hello-world test again, you will pull the image from the repository, because it doesn't exist locally anymore. And if you remove the container before removing the image, you won't need the force-remove anymore, either.