fix(docs): add missing Docker guide

Co-authored-by: Simon Legner <Simon.Legner@gmail.com>

.github/CONTRIBUTING.md

@@ -80,6 +80,23 @@ Follow the following steps to update documentations to their latest version:
 * no trailing whitespace; blank lines should have no spaces; new line at end-of-file
 * use the same coding style as the rest of the codebase
 
+## Developing in Docker
+
+To set up a Docker development enviroment:
+
+1. Install [Docker](https://docs.docker.com/get-docker/) and [Docker Compose](https://docs.docker.com/compose/install/).
+2. Navigate to the root of this repository.
+3. `cp sample.env .env`
+4. `docker-compose up -d`
+5. `docker-compose run --rm --service-ports devdocs`
+  
+You should now be in a bash shell inside the Docker container.  The repo is mounted at `/home/devuser`, so any changes you make there will be reflected on the host (your machine) and vice versa.  Commands can all be run inside the container:
+
+1. `thor docs:download --default` to get the default docs.
+2. `rackup -o 0.0.0.0` to start the server
+
+Navigate to `http://localhost:9292` to see the running site.
+
 ## Questions?
 
 If you have any questions, please feel free to ask them on the contributor chat room on [Gitter](https://gitter.im/FreeCodeCamp/DevDocs).