adding documentation on how to build the image yourself

docker/README.md

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+Building Docker Image
+=====================
+
+This directory contains the necessary Dockerfile and wrapper script for building a Docker Image. This is a Linux based image that builds and runs choco.exe with mono.
+
+To build this image yourself, follow these steps:
+
+1. Clone down the repository using `git clone https://github.com/chocolatey/choco.git`.
+1. Change directories to the root of the repository.
+1. Run the docker build command. `docker build -t mono-choco -f docker/Dockerfile.linux .` (the trailing . is important)
+1. Run your new image using the command `docker run -ti --rm mono-choco /bin/bash`
+1. Test choco by running `choco -h`. You should see the help message from choco.exe.