Apt-cacher example broken

from example https://docs.docker.com/engine/examples/apt-cacher-ng/

Can anyone diagnose the problem why this example fails. I build it with the exact Dockerfile and the exact run command as shown. The container appears to start and shuts down immediately.

“docker logs -f test_apt_cacher_ng” will open the container long enough to spit out the following errors.

  • Starting apt-cacher-ng apt-cacher-ng
    WARNING: No configuration was read from file:sfnet_mirrors
    …done.
    tail: error reading ‘/var/log/apt-cacher-ng/’: Is a directory
    tail: /var/log/apt-cacher-ng/: cannot follow end of this type of file; giving up on this name
    tail: no files remaining

Docker version 17.12.0-ce, build c97c6d6
Ubuntu 16.04.3 LTS (GNU/Linux 4.4.0-104-generic x86_64)

I seems that docker would be much easier to learn with working example.