Added Dockerfile

[squarepear]

Added a Dockerfile so you can run the client in Docker

I also added a NAME env variable to use instead of the current username when detected. Docker wouldn't pick up the user's name, so this should fix that.

[quackduck]

This is very nice! If I understand correctly, the necessary config files will be copied in automatically?

There's just one thing that'll make the docker way different from the regular way:
Duckcoin rn does not use your username as the name sent in blocks, but rather your full name as defined in your computer profile.

Another thing, since this won't work on Windows (the $USER variable is not set on Windows), I'm not sure why I would add the Docker config? (I'm super new to Docker)