runsc inside of default docker seccomp policy

@scanlime on Twitter is trying to [run runsc inside a Docker container](https://twitter.com/scanlime/status/1310630296694398977) with the standard seccomp [policy](https://github.com/moby/moby/blob/master/profiles/seccomp/default.json) enabled. This is similar to rootless mode (#311), but a little bit more strict.

The immediate issue is that we exec into empty namespaces, which the profile does not allow. It is not clear if there would be more issues if that were resolved, though I didn't see any glaring issues comparing our seccomp filters to Docker's.

It's also not clear if the defense-in-depth features we'd have to disable to make this work would make it a bad idea. But in general, it is very reasonable to want to run a sandbox as a subprocess in an existing container.

cc @fvoznika @nlacasse