Conditional screens directory

To try and address the points raised in https://openproject.helmholtz-berlin.de/notifications/details/2534/activity I made the directory loaded by the container in the start.sh script take an environment variable.

If the environment variable PHOEBUS_SCREENS_DIR is set then this macro will be used as the directory to be mounted. Otherwise the gui directory cloned by default with the repo is used.

I did this because I want this repo to work out of the box even if it's not used with ansible.