[docker-compose] configure Identity endpoint for backup in commtest

Summary:
ENG-6145.

We need this to test querying userID based on user identitifier.

Depends on D13806

Test Plan: IDENTITY_SERVICE_ENDPOINT defined when running commtest in docker

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13817