list base64 as a runtime dependency

Since Ruby 3.4, base64 is no longer a default and needs to be installed explicitly: https://bugs.ruby-lang.org/issues/20187

Author: razumau

docker-api.gemspec

@@ -14,6 +14,7 @@ Gem::Specification.new do |gem|
   gem.version       = Docker::VERSION
   gem.add_dependency 'excon', '>= 0.64.0'
   gem.add_dependency 'multi_json'
+  gem.add_dependency 'base64'
   gem.add_development_dependency 'rake'
   gem.add_development_dependency 'rspec', '~> 3.0'
   gem.add_development_dependency 'rspec-its'