Sure! thanks for offering. Please feel to work on it.

The "container" and "task" below are referring to containerd concept: https://github.com/containerd/containerd/blob/main/docs/getting-started.md#creating-a-running-task

--attach comes into effect in when creating new task today. https://github.com/containerd/nerdctl/blob/main/pkg/taskutil/taskutil.go#L43

nerdctl start passes the --attach flag when creating new task.
nerdctl run does not support --attach flag and hardcoded it to false when creating new task. It should be easy to support --attach in nerdctl run. I may also complete it in a separated PR.
nerdctl create only creates container without task. That requires persistently storing --attach info in container.

The additional --attach information will be added to container struct in containerd and stored in container store.

If --attach is also included in nerdctl start for this container, the task will be created with the one in nerdctl start. Otherwise, it will use the one in container store.

The only benefit I can think of adding --attach in nerdctl create is to make it more docker-compatible. The functionality seems already covered by --attach flag in nerdctl create.

Do you think it is good to add --attach in nerdctl create with the above approach? Or is it better to save the duplicate functionality even it is not Docker-compatible? Let me know the tenets you prefer to follow, or if you have better ideas or concerns.

cc @AkihiroSuda as you reviewed the previous PR to add --attach in nerdctl start

The additional --attach information will be added to container struct in containerd and stored in container store.

Do you mean add a new field to container struct or utilize existing fields? I think you can utilize Labels for this (or maybe Extensions? I don't know much about Extensions but from its comment Extensions stores client-specified metadata). You can add a label for flagA during create and check its value during start.

The only benefit I can think of adding --attach in nerdctl create is to make it more docker-compatible. The functionality seems already covered by --attach flag in nerdctl create.

That's also what I thought (since nerdctl create+nerdctl start -a = nerdctl create -a + nerdctl start). I don't have strong preference on whether or not to have the feature. The above label method should not add much code anyway :)