Unset bootstrap credentials before exec-ing the server (#1412)

POSTGRES_PASSWORD (and related vars) are only needed during initdb and
the temporary-server initialisation phase. After that they serve no
purpose, but remain in the process environment for the entire lifetime
of the container, where any loaded C extension can read them via
environ.

Unsetting them immediately before the final exec ensures the running
PostgreSQL server process starts with a clean environment.

Author: mkindahl