Fix exim container configuration

Summary:
Fixes 501-5.5.4 HELO/EHLO argument "container['host']" invalid from gmail.

This is a follow-up for 3c52ac6a0705 and 24aa4870b750.

Ref T1360.

Test Plan: After the container has been reprovisioned, mail has been received.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1360

Differential Revision: https://devcentral.nasqueron.org/D2010