Vagrant box creation fails: can't unmount chroot that's busy

Originally created by @intrigeri on #12530 (Redmine)

Optimizing image for compression
Umounting /tmp/tmpvsGC26
EEEK! Something bad happened...
command failed: ['umount', '/tmp/tmpvsGC26']

umount: /tmp/tmpvsGC26: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

Cleaning up
ERROR: /tmp/tmpvsGC26: Device or resource busy
rake aborted!

So I can’t build any ISO since the last changes to our build system were merged.

It’s kept busy by /tmp/tmpvsGC26/usr/sbin/sshd. Perhaps we need to stop the sshd service in the chroot at some point? But really, sshd should not even start in a chroot in the first place, so apparently something else is wrong, and it might be worth investigating to fix the root cause of the problem instead of workaround’ing it.

Parent Task: #5630 (closed)