[systemd-devel] What makes systemd-nspawn "not suitable for secure container setups"?

[Josh Triplett]

The systemd-nspawn manpage lists the various mechanisms used to isolate
the container, and then says "Note that even though these security
precautions are taken systemd-nspawn is not suitable for secure
container setups. Many of the security features may be circumvented and
are hence primarily useful to avoid accidental changes to the host
system from the container."

How can a process in a systemd-nspawn container circumvent the container
setup?  What additional steps would systemd-nspawn need to take to
provide a secure container setup?

- Josh Triplett