Linux does offer enough primitives to completely sandbox processes, as Zones and Jails did/do. The problem is that docker didn't implement all of those primitiv

Linux does offer enough primitives to completely sandbox processes, as Zones and Jails did/do. The problem is that docker didn't implement all of those primitives right away, the biggest being user namespacing, which it didn't add until shockingly late in its development lifecycle. So, yes you can completely sandbox a "container", just be prepared to put in some work (SELinux, AppArmor, user namespaces, and reviewing the defaults capabilities quite carefully).