add entrypoint.sh script to certificate_handler

Moves commands defined in the command section in the different compose files into a single shell script that is copied at creation into the certificate_handler container and servers as the entrypoint / startup script for the container.

The commands have been copied without any alteration except that there are now not chained together. Therefore in case an error occurs we will see exactly which command threw the error:

e. g. chmod command on wrong file:

e. g. missing certificates:

The errors provided seem to be sufficient for debugging. No additional "safety controls" have been added to the script.