Add initial idris Dockerfile

Partially inspired by https://github.com/darinmorrison/docker-haskell/blob/da366ea78e9e9db3caa3fda79e9864cb0a31db0b/7.8/Dockerfile

Partially follows:
* https://github.com/idris-lang/Idris-dev/wiki/Idris-on-Debian
* https://github.com/idris-lang/Idris-dev/wiki/Idris-on-Ubuntu
* https://github.com/idris-lang/Idris-dev/wiki/Idris-on-OS-X-using-Homebrew