Better Dockerfile support
Merge branch 'better_docker' of https://github.com/pjz/mailinabox into pjz-better_docker

our trees had diverged, various conflicts resolved
docker: fix startup scripts for nsd and dovecot to run them in the foreground
system.sh: make apt-get upgrade quieter
docker: don't start services ourself

* let the base image's system services manager handle it
* move our container start script to occur before system services are started
missing wget dependency used by roundcube installation
test_mail: gracefully handled when the server has no reverse DNS available
merge: @PirosB3 and @pjz suggested using pjzz/phusion-baseimage as the base image for docker

See http://phusion.github.io/baseimage-docker/ for why the stock Ubuntu
image from Docker is not good enough for a complex system.

Thanks to @pjz for updating the base image for Ubuntu 14.04 and starting
the service scripts.

see #16; merges #49
DKIM: For the benefit of ADSP and DMARC (not yet impl), each sending domain should be its signing domain
add ADSP and DMARC records; see #14
test_dns: handle case where a DNS record is missing (vs incorrect)
test_dns: properly test the non-primary domain of a box (for email addresses on domains besides PUBLIC_HOSTNAME)
test_dns: add ADSP and DMARC tests, see #14