Keep tag of indirect used images while docker image prune -a

[PMExtra]

Steps to reproduce the issue:

1. Build a local image that FROM image_foo
2. Run a container with the local image
3. Execute docker image prune -a

Describe the results you received:

The image_foo will be untagged.

Describe the results you expected:

The image_foo cannot be truly removed, because it's an indirect depend of the running container.

So I think we can keep the tag.

Generally, we use prune because we want to free up storage. But removing a tag doesn't work for the goal. It just made more dangling images and more confusing.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client: Docker Engine - Community
 Version:           20.10.11
 API version:       1.41
 Go version:        go1.16.9
 Git commit:        dea9396
 Built:             Thu Nov 18 00:36:58 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.11
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.16.9
  Git commit:       847da18
  Built:            Thu Nov 18 00:35:20 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.12
  GitCommit:        7b11cfaabd73bb80907dd23182b9347b4245eb5d
 runc:
  Version:          1.0.2
  GitCommit:        v1.0.2-0-g52b36a2
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

[thaJeztah]

Thanks for your proposal.

The image_foo cannot be truly removed, because it's an indirect depend of the running container.

The relation in this case would only be when using the "classic" builder, and only for the build-cache. The image that's produced does not have a relation with the original image (other than sharing the same layers). When using BuildKit as builder (DOCKER_BUILDKIT=1) the build-cache is fully separate from the image-cache, so there won't be a direct relation between the two (even for build).

That said, you can label the original image, and use that to exclude it from being pruned, and there are some proposals, such as https://github.com/docker/cli/issues/3557 and https://github.com/docker/cli/issues/3027 (some other ones as well) to provide more options on "filtering" what will be pruned.