Add docker-compose.yml file

To run the site in docker is now a single command.

index.md

@@ -74,16 +74,11 @@ If you want to shorten this long command, you can put this code in a file such a
 ## Running the site in Docker
 
 You can also use Docker to directly build and run the site on your local machine. Just clone the repo and run the following from your working dir:
+```
+docker-compose up -d
+```
+The site should now be running at [http://localhost:4000/](http://localhost:4000/).
 
-```
-docker build --no-cache -t mydocs .
-```
-
-Once the build is complete, you can mount and run the whole site as follows:
-
-```
-docker run -v "$PWD:/src" -p 4000:4000 mydocs serve -H 0.0.0.0
-```
 This is perhaps the easiest way to see how your site would actually look.
 
 ## Configure the sidebar