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