docker: do ADD container/docker later on so that the Dockerfile can be updated and still reuse a cached image after the major setup steps are done

docker: if container was launched with a tty start bash otherwise loop forever to keep the container going

don't reject mail to domains that only have aliases and not users

thanks @konklone for discussion

move the Dockerfile to the root to allow the working directory of the repo to be pushed inside the image (rather than inside the container getting a fresh mailinabox from github) so changes in the working copy can be tested in Docker quickly / without pushing to github

We were installing .deb's from Debian. The next apt-get upgrade would cause roundcube to be upgraded.
Maybe that also triggered the installation of apache. Now install roundcube from Ubuntu. So long as
Ubuntu doesn't post an update to roundcube, at least it won't trigger an upgrade on the next
apt-get upgrade. This should also mean we don't need to purge the installation of apache2.

Also try using apt-mark hold to prevent roundcube from being updated, in case that will trigger
dependencies.

make a bash function to use everywhere we apt-get-install (`DEBIAN_FRONTEND=noninteractive apt-get -qq -y `)

ensures the output is quiet

enable roundcube's password-change plugin

add warnings about using a fresh machine because I wont support installing this on an existing machine

List a DKIM and SPF testing tool in the documentation.

Seems like it was removed in nsd4 (since Ubuntu 14.04). It now
silently does nothing.

Fixes #41.

http://pingability.com/zoneinfo.jsp for checking DNS and MX servers.

Added DKIM and SPF testing tool to 'Checking that it worked' section.

move the ssh PasswordAuthentication check to after installing ssh

instead of installing roundcube from Ubuntu (before replacing it), just install the dependencies we need

I had previously awkwardly disabled it because on my VPS the necessary kernel
module wasn't available. I meant to make it an option, but instead I took
out the apt-get install. Now it works for me so I'm putting everything back.

Force the scripts to run in bash

nsd doesn't install without creating the `nsd` user first - merge branch from randallsquared-master, fixes #25