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/overlays/default.nix

@@ -1,5 +1,6 @@
 {
   gitignore = import ./gitignore.nix;
+  ghr = import ./ghr;
   haskell-packages = import ./haskell-packages;
   postgresql-default = import ./postgresql-default.nix;
   postgresql-legacy = import ./postgresql-legacy.nix;

nix/overlays/ghr/default.nix

@@ -0,0 +1,7 @@
+self: super:
+# Overlay that adds `ghr`: Upload multiple artifacts to GitHub Release in
+# parallel, http://tcnksm.github.io/ghr/
+
+{
+  ghr = super.callPackage ./ghr.nix { };
+}

nix/overlays/ghr/ghr.nix

@@ -0,0 +1,15 @@
+{ buildGoModule, fetchFromGitHub }:
+
+buildGoModule rec {
+  pname = "ghr";
+  version = "0.13.0";
+
+  src = fetchFromGitHub {
+    rev = "v${version}";
+    owner = "tcnksm";
+    repo = "ghr";
+    sha256 = "1nm5kdjkqayxh06j9nr5daic9sw9nx9w06y9gaqhdrw9byvjpr1a";
+  };
+
+  vendorSha256 = "14avsngzhl1b8a05i43ph6sxh9vj0jls0acxr9j7r0h3f0vpamcj";
+}