Making it possible to vary the Mujina repo URL

* Adding ENV MUJINA_GIT_URL with a sensible default to the Dockerfile
* Move cloning and building Mujina out of the Dockerfile
* Build and run Mujina at runtime using a shell script
* Adding the /mujina-idp.sh shell script
Changing the documentation
Fixing the build script

Tag the image both with the current version and 'latest'.
Adding to the documentation

* Example on setting the GIT url
* Example on changing the host port