Without exec the `postgrest` process is not run with PID 1 (it
is a child process of the shell that starts it). This means
signals send to the docker (like `docker stop` or ^C) will
not be handled correctly.
However, Linux treats PID 1 as special and sets the SIGTERM
handler to ignore by default. It is also necessary to install
a SIGTERM handler.
This commit adds `exec` to resolve this problem, as per the
recommendation in the Dockerfile documentation:
https://docs.docker.com/engine/reference/builder/#shell-form-entrypoint-example
Joe Nelson