Use preferred exec form for CMD

Summary:
This is a follow-up for 1d59efe54d37.

Ref T1791

Test Plan: docker run -e POSTGRES_PASSWORD=sentry <hash> on Dwellers

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1791

Differential Revision: https://devcentral.nasqueron.org/D2866