Add docker-compose and Dockerfile installations

[jeremyll]

Should be able to run the send text demo within 2 commands:

sudo docker-compose run --rm flaschen make all
sudo docker-compose run --rm flaschen ./send-text -f fonts/6x10.bdf -o "Nice work"

[hzeller]

Mmh, not sure if it is very useful to have the client programs dockerified; can you explain why you think that would be useful ?

I'd be rather interested in a dockerified version of the server, as it (a) needs to access GPIO and (b) might create real-time threads and (c) listens on ports (though all unprivileged). Right now it does the usual root/drop-priv/become-daemon spiel, but having that in a docker container might be an interesting exercise.