Nixify CircleCI setup (#1535)

* package ghr
* add nix-based release scripts
* refactor CI based on nix tests and release scripts
* update stack version in circleci
* update makefile
* remove docker directory
* add Docker Hub description
* add README on docker

nix/release/docker-hub-full-description.md

@@ -0,0 +1,33 @@
+# PostgREST
+
+[![Join the chat at https://gitter.im/begriffs/postgrest](https://img.shields.io/badge/gitter-join%20chat%20%E2%86%92-brightgreen.svg)](https://gitter.im/begriffs/postgrest)
+[![Donate](https://img.shields.io/badge/Donate-Patreon-orange.svg?colorB=F96854)](https://www.patreon.com/postgrest)
+[![Donate](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.me/postgrest)
+[![Docs](https://img.shields.io/badge/docs-latest-brightgreen.svg?style=flat)](http://postgrest.org)
+[![Build Status](https://circleci.com/gh/PostgREST/postgrest/tree/master.svg?style=shield)](https://circleci.com/gh/PostgREST/postgrest/tree/master)
+
+PostgREST serves a fully RESTful API from any existing PostgreSQL database. It
+provides a cleaner, more standards-compliant, faster API than you are likely to
+write from scratch.
+
+# Usage
+
+To learn how to use this container, see the [PostgREST Docker
+documentation](https://postgrest.com/en/stable/install.html#docker).
+
+You can configure the PostgREST image by setting the enviroment variables used
+in the default `/etc/postgrest.conf` file or overriding that file. This is the
+default configuration file:
+
+```
+$DEFAULT_CONFIG
+```
+
+# How this image is built
+
+The image is built from scratch using
+[Nix](https://nixos.org/nixpkgs/manual/#sec-pkgs-dockerTools) instead of a
+`Dockerfile`, which yields a higly secure and optimized image. This is also why
+no commands are listed in the image history. See the [PostgREST
+respository](https://github.com/PostgREST/postgrest/tree/master/nix/docker) for
+details on the build process and how to inspect the image.