Feature Request: Attach to specific process ID inside Docker container

Description

As of today, the Docker CLI allows you to attach to a running container's entrypoint process. However, it is not possible to attach to a specific process inside a container.

Steps to reproduce the issue:

1. Spin up a MySQL container docker run --detach --env MYSQL_ROOT_PASSWORD=blah --pull always mysql:latest
2. Create an interactive Bash process running in the same container docker exec --interactive --tty <containerId> bash
3. Detach from the Bash shell with CTRL + P, CTRL + Q
4. Inspect processes inside container docker top <containerId>
5. Notice that Bash process is still running, we've just detached from it.
6. Attach to the Bash process inside the container .... this capability doesn't exist.
7. I would imagine being able to run a command similar to: docker attach <containerId> <processId>

Output of docker version:

Client:
 Cloud integration: v1.0.20
 Version:           20.10.10
 API version:       1.41
 Go version:        go1.16.9
 Git commit:        b485636
 Built:             Mon Oct 25 07:47:53 2021
 OS/Arch:           windows/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.10
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.16.9
  Git commit:       e2f740d
  Built:            Mon Oct 25 07:41:30 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.11
  GitCommit:        5b46e404f6b9f661a205e28d59c982d3634148f8
 runc:
  Version:          1.0.2
  GitCommit:        v1.0.2-0-g52b36a2
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Build with BuildKit (Docker Inc., v0.6.3)
  compose: Docker Compose (Docker Inc., v2.1.1)
  scan: Docker Scan (Docker Inc., 0.9.0)

Server:
 Containers: 6
  Running: 3
  Paused: 0
  Stopped: 3
 Images: 10
 Server Version: 20.10.10
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 5b46e404f6b9f661a205e28d59c982d3634148f8
 runc version: v1.0.2-0-g52b36a2
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 5.10.60.1-microsoft-standard-WSL2
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 6
 Total Memory: 15.59GiB
 Name: docker-desktop
 ID: LUJQ:6XRO:QAGY:NC7E:DBP2:HH3H:TIA6:Q3TN:U53M:4MH3:PO3R:WI26
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Start with the docker attach command and reproduce the documented MySQL and Bash process steps. Investigate how a process ID could be selected inside a container, including the proposed command syntax; done means a specific in-container process can be attached to and the behavior is covered by appropriate CLI tests.