Build Mono runtime for use with the Heroku .NET buildpack.

To build:

$ docker build -t my-user/mono-builder cedar-14/.

To build and upload new Mono version:

$ docker run -v ${PWD}/cache:/var/cache -v ${PWD}/out:/var/out -e VERSION=4.0.1 -e STACK=cedar-14 my-user/mono-builder