Allow Docker containers to use ACME DNS API

Summary:
On the Docker engines, the certbot command is run as a container,
and so doesn't use public IP.

This is a follow-up for efa09f17.

Test Plan: Tested on Equatower with D2181.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2182