Resolve "rake system_tests freezes in CI"

Closes #612 (closed).

Required:

• Change server to pull images from to https://images.linuxcontainers.org/. The old one was returning lookup us.images.linuxcontainers.org: no such host.

Optional:

• /etc/setgid typo. Found it while I was doing my local setup.
• Print exceptions. They would have been very helpful in determing the source of error now.