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

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

Start at the docker image prune -a command entry point and reproduce the reported sequence with a local image based on image_foo and a running container. Trace how indirect image dependencies are considered during pruning. Done means the indirectly used image_foo tag is retained while unrelated reclaimable images can still be pruned.