Prevent accessing the external network (a.k.a. INET) when building

Comparing to a local build based on sbuild (that prevents the build process to access the network), this tango build job should trigger WARNING because it cannot reach Internet:

loading intersphinx inventory 'itango' from https://itango.readthedocs.io/en/latest/objects.inv ...
loading intersphinx inventory 'pytango' from https://pytango.readthedocs.io/en/latest/objects.inv ...
loading intersphinx inventory 'python' from https://docs.python.org/3/objects.inv ...
WARNING: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://itango.readthedocs.io/en/latest/objects.inv' not fetchable due to <class 'requests.exceptions.ProxyError'>: HTTPSConnectionPool(host='itango.readthedocs.io', port=443): Max retries exceeded with url: /en/latest/objects.inv (Caused by ProxyError('Unable to connect to proxy', NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7f69e7bc9400>: Failed to establish a new connection: [Errno 111] Connection refused')))
...

The pipeline should mimic the default debian building behaviour regarding network access.