Add dockerfile & docker-compose

[cghislai]

Here is a dockerfile i thought mught be worth sharing.

You can setup a account on hub.docker.com to build it on each commit. Users then would be able to copy/adapt the provided docker-compose.yml and deploy a spads service in no time.

There are some issue though:

• non-interactive mode for the installer is a pain
• credentials are passed to the command line
• game/maps have to be provided manually
• bind-mounting an host directory will 'hide' the image-provided content (using docker volumes works though).
• host networking mode seems required for the server to be reachable, even if its ip is specified in the config. Spads seems to use a guessed one which is incorrect in nated networks.