add Dockerfile

[xgaia]

Hi @grempe,

I add a Dockerfile to ease the deployment.

docker run -d -p 8008:80 --name diceware xgaia/diceware

Go to localhost:8008 and roll the dice !

dice.libreon.fr is using this docker image.

If you validate the PR, could you link the repo to Dockerhub ? This way the image can be retrieved by doing docker pull gremp/diceware:latest.

Thanks !