Updated the Docker script to start the site correctly. The DIR variable was not being used in each place which it should have been used and this has allowed the removal of the `readlink -f` command which had compatibility issues with MacOS (#297).
The README did not have the change in name for the docker script reflected in all references so this has also been corrected.
Developing using Docker is now simple and avoids the need to install PHP on the developer's host machine. Common code from `dev.sh` and `dev-docker.sh` has been extracted into a `dev-prepare.sh`. README contains instructions on how to use both dev scripts - which the developer can choose based on their preference.
