Revert to hackney ~> 1.16: 4.x cannot use unix sockets

mix.exs

@@ -47,9 +47,20 @@ defmodule Excontainers.MixProject do
       # {:dep_from_hexpm, "~> 0.3.0"},
       # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
 
-      # Only ever reached through Tesla.Adapter.Hackney, so we mirror Tesla's own
-      # supported range: hackney 2.x/3.x/4.0.0/4.0.1 lack exports Tesla relies on.
-      {:hackney, "~> 1.16 or ~> 4.0 and >= 4.0.2"},
+      # Pinned to 1.x on purpose: we reach the Docker daemon over a unix domain
+      # socket (`http+unix://%2Fvar%2Frun%2Fdocker.sock`, see Docker.Api.HackneyHost)
+      # and hackney 4.x cannot connect to one. 4.x still parses the URL correctly
+      # and ships an identical hackney_local_tcp, but its rewritten connection
+      # layer sends the request through hackney_happy (happy-eyeballs DNS) and
+      # never dispatches the local_tcp transport:
+      #
+      #   :hackney.request(:get, "http+unix://%2Fvar%2Frun%2Fdocker.sock/v1.41/version", [], "", [])
+      #   1.25.0 => {:error, :enoent}    (tried the socket)
+      #   4.7.2  => {:error, :nxdomain}  (tried DNS)
+      #
+      # Swapping adapters is not an option either — hackney is the only Tesla
+      # adapter with unix-socket support. Revisit once upstream restores it.
+      {:hackney, "~> 1.16"},
       {:jason, ">= 1.0.0"},
       {:tesla, "~> 1.4"},
       {:gestalt, "~> 1.0 or ~> 2.0"},