Rollback cowboy dep to v2.10.0

Upgrade to v2.12.0 yields an error like "Response cookies must be set
using cowboy_req:set_resp_cookie/3,4." which seems to be caused by a
version mismatch between cowboy and plug_cowboy. To be investigated
further, but at least we have a working image again now.

mix.exs

@@ -31,6 +31,7 @@ defmodule Proxy.Mixfile do
   defp deps do
     [{:plug_mint_proxy, git: "https://github.com/madnificent/plug-mint-proxy.git", tag: "v0.2.0"},
      {:uuid, "~> 1.1"},
+     {:cowboy, "~> 2.10.0"},
      {:replug, "~> 0.1.0"},
      {:secure_random, "~> 0.5"},
      {:observer_cli, "~> 1.5"}]