Support --sig-proxy in run and attach

[Channing Gaddy]

What is the problem you're trying to solve

--sig-proxy is supported in docker run and docker attach but not in nerdctl run and nerdctl attach. This can also help to unblock #392

Describe the solution you'd like

Support --sig-proxy in nerdctl run and nerdctl attach.

Additional context

• --sig-proxy is listed as unimplemented under nerdctl run and nerdctl attach

• Docker attach docs

• Docker run docs