[Enhancement] Improve Dockerfile permissions (#157)

* Updated dockerfile

* added a healthcheck

test/pinchflat_web/controllers/health_controller_test.exs

@@ -0,0 +1,10 @@
+defmodule PinchflatWeb.HealthControllerTest do
+  use PinchflatWeb.ConnCase
+
+  describe "GET /healthcheck" do
+    test "returns ok", %{conn: conn} do
+      conn = get(conn, "/healthcheck")
+      assert json_response(conn, 200) == %{"status" => "ok"}
+    end
+  end
+end