[Enhancement] Improve Dockerfile permissions (#157)

* Updated dockerfile

* added a healthcheck

lib/pinchflat_web/controllers/health_controller.ex

@@ -0,0 +1,9 @@
+defmodule PinchflatWeb.HealthController do
+  use PinchflatWeb, :controller
+
+  def check(conn, _params) do
+    conn
+    |> put_status(:ok)
+    |> json(%{status: "ok"})
+  end
+end