Add instruction for custom root certificate

[agutenkunst]

Add section to doc describing how to run industrial_ci locally if a root certificate needs to be used.

I am aware of DOCKER_RUN_OPTS="-v /etc/ssl/certs:/etc/ssl/certs:ro" also mentioned in the Readme.
This however did not work for me since the certificate installation on the local machine would fail during the
init-step with

Setting up ca-certificates (20210119~20.04.1) ...
Updating certificates in /etc/ssl/certs...
rm: cannot remove 'ca-certificates.crt': Read-only file system
dpkg: error processing package ca-certificates (--configure):
 installed ca-certificates package post-installation script subprocess returned error exit status 1

...
Errors were encountered while processing:
 ca-certificates
E: Sub-process /usr/bin/dpkg returned an error code (1)
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
'init' returned with code '100' after 1 min 19 sec
Tests failed exit code '100'

this seems to be due to the existing ca-certificates.crt which originates from the installation of the certificate on the local system.

Not sure if to many have this problem but I think it is worth writing down the solution.

Open for suggestions / better solutions!