Use CMD exec in Dockerfile

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

CHANGELOG.md

@@ -6,7 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## Unreleased
 
 ### Fixed
-- Terminate gracefully on SIGTERM - @recmo
+* Terminate gracefully on SIGTERM (for use in Docker) - @recmo
 
 ## [0.3.1.0] - 2016-02-28