There are arguments - which are at least plausible - that the same effort is better spent on VM / Solaris Zones / FreeBSD jails / traditional chroot / SELinux /

There are arguments - which are at least plausible - that the same effort is better spent on VM / Solaris Zones / FreeBSD jails / traditional chroot / SELinux / just using dedicated hardware for everything / ...; several of these have the advantage of not encouraging people to throw up a compartment (of some kind) and never update it again, which is certainly something that happens with Docker.