Resolve Gitea Docker gateway without iproute2

scripts/private_default_gateway.rb

@@ -0,0 +1,28 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "ipaddr"
+
+route_path = ARGV.fetch(0, "/proc/net/route")
+routes = File.foreach(route_path).drop(1).filter_map do |line|
+  fields = line.split
+  next unless fields.length >= 8
+  next unless fields.fetch(1) == "00000000"
+  next unless (fields.fetch(3).to_i(16) & 0x2) == 0x2
+
+  fields
+end
+
+routes.sort_by { |fields| fields.fetch(6).to_i }.each do |fields|
+  gateway_hex = fields.fetch(2)
+  next unless gateway_hex.match?(/\A[0-9A-Fa-f]{8}\z/)
+
+  gateway = gateway_hex.scan(/../).reverse.map { |octet| octet.to_i(16) }.join(".")
+  address = IPAddr.new(gateway)
+  next unless address.ipv4? && address.private?
+
+  puts address
+  exit 0
+end
+
+abort "no private IPv4 default-route gateway found in #{route_path}"