Adds Dockerfile

[EnigmaCurry]

This adds a Dockerfile and a make docker target to build tty-clock in an alpine container image.

With the Dockerfile colocated with the source code in git, you can build the image all with one command:

## Build using my fork (replace /xorg62 if merged):
docker build https://github.com/EnigmaCurry/tty-clock.git#master -t localhost/tty-clock

## To run the image:
docker run --rm -it localhost/tty-clock tty-clock -u -c -s -C 3 -b

[EnigmaCurry]

I forgot that inside the container it was always thinking the timezone was UTC, so I added the TIMEZONE build argument to set it how you like.

DOCKER_TIMEZONE=America/Los_Angeles DOCKER_IMAGE=xorg62/tty-clock make docker

[anarcat]

why isn't this just a TZ environment that can be passed by the docker run caller?

[EnigmaCurry]

ah, that works too, you can set the default for the container with the build argument, as well as override it at runtime with the environment TZ. I could remove the default, but I think its nice to be able to set it and not have to specify it again.