Use gaol or seccomp for sandboxing

[Manishearth]

http://security.stackexchange.com/a/107853/7497

Docker has a blacklist approach for security, which isn't ideal. We should be using a whitelisting sandbox.

[DemiMarie]

This is why I think the move from playpen to Docker was a mistake. Playpen is expressly designed to run totally untrusted code. Docker isn't.

Nevertheless, Docker supports seccomp, and we should be using it.