Adding a docker environment

[Billiam]

Adds a basic Dockerfile with lein exec.

Can be run via repl like the readme, or with

docker run -it --rm -v "$(pwd)/things:/usr/src/app/things" -v "$(pwd)/src:/usr/src/app/src" dactyl"

(but not as quickly).

[cloudbring]

I actually just did this and was about to edit the readme and make a pull request.