Make systemd call entrypoint.py

[ckunki]

Ticket #69 described adding an entry point to the DSS Docker Container Images in order to copy notebook files and start the jupyter server.

The entrypoint was implemented as Python script entrypoint.py.
The current ticket requests to call entrypoint.py for other image types (AMI, vmdk, vdh), too.

Please note: This is not required for calling magic %pip, see #273