Fix Gitian instructions to setup LXC container networking on Debian

Summary:
The current documentation was tested successfully when packets are
cached locally, but the LXC container couldn't any external domain.
After some investigation, the bridge was not linked to the active
network connection by the rc.local script.
This diff adds the instructions to do so.

Test Plan:
Setup the Gitian environment following the documentation.
Check your host can ping google.com:

ping -c 2 google.com

If the ping is not successful, fix you host network connection, else:

cd gitian-builder
sudo lxc-execute -n gitian -f var/lxc.config -- sudo -u root ping -c 2
google.com

Check the ping is successful.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3187