Fix IPv6 Docker support to access lab-pengutronix

[gctucker]

Now that all Jenkins jobs are running within Docker, there appears to be an issue with routing IPv6 traffic to external servers. This is an issue in particular with lab-pengutronix which supports IPv6 only:
https://hekla.openlab.pengutronix.de/

The issue appears to be in the configuration of the builder itself rather than in the Docker container. Routing IPv6 from a Docker container also seems to be a non-trivial thing to configure, it probably needs someone to spend a couple of hours to get this set up correctly and add it to the Ansible builder config.

[gctucker]

The Python error when trying to access lab-pengutronix is:

  File "/usr/lib/python3.7/socket.py", line 727, in create_connection
    raise err
  File "/usr/lib/python3.7/socket.py", line 716, in create_connection
    sock.connect(sa)
OSError: [Errno 99] Cannot assign requested address

[jluebbe]

We've now added a proxy which has an IPv4 address, so it should now be accessible via both protocols.