Create docker image so people can easily setup their own mail-server

[jaytaph]

We should make a simple docker container for the mail server.

We basically have this already running (bitmaelum/bitmaelum-suite).. but it might need some work to make sure it's ready for use.

[jaytaph]

What needs to be done:

• [ ] have a single docker container with all tools installed in /usr/bin
• [ ] sort of bootstrap on first run
• [ ] define configuration environment settings, like BITMAELUM_HOST etc, which gets added to the default config
• [ ] allow users to use their own custom config (mount volume to /etc/bitmaelum/server-config.yml)
• [ ] auto-detect external IP (ipv4bot.whatismyipaddress.com)
• [ ] define the rest of the todos

[jaytaph]

related to #139