[build] Add Docker image files

This commit adds files necessary to build a working Docker image that
runs `realmd` + `mangosd` supervised by `supervisord`.

The Docker image expects three volumes to be mounted into the container:

* /opt/mangos/data - (required) must contain DBCs, maps etc.
* /opt/mangos/etc - (required) must contain configuration files
* /opt/mangos/logs - (optional) stdout/stderr and all logs will be written
  to this folder. If you want to collect them off the container, use a volume.

MaNGOS is compiled and installed into `/opt/mangos`.

Using the image is very simple. Assuming you've set your MySQL
configuration to use `localhost` (using socket connections), you could
run this image as follows:

    docker run --restart="always" --name mangos-server \
        -v /var/lib/mysql/mysql.sock:/var/run/mysqld/mysqld.sock \
        -v ~/mangosdata:/opt/mangos/data \  # DBCs etc. in ~/mangosdata
        -v ~/mangosconf:/opt/mangos/etc \
        -v ~/mangoslogs:/opt/mangos/logs \
        -p 3724:3724 -p 8085:8085
        -ti tazjin/mangoszero

And you would instantly have a working world and login server.