Added documentation for editing the site using Docker

Docker.md

@@ -0,0 +1,22 @@
+# Editing the site using Docker
+
+It is also possible to work on the site using Docker. The process is as
+follows:
+
+* Build the Docker image 
+    docker build . -t perlwebadvice
+
+* Run the Docker container 
+    docker run -p7007:7007 -v$PWD:/perlwebadvice perlwebadvice
+
+* You can then see a local version of the site at http://localhost:7007/
+
+* In a different command terminal run `docker ps` to see the name of your container
+
+* When you have changed something, you can rebuild the site by running 
+    docker exec YOUR_CONTAINER_NAME aphra build
+
+* Your new version of the site will appear when you refresh http://localhost:7007/
+
+* To close down the Docker container, run 
+    docker stop YOUR_CONTAINER_NAME