Stop using the Cane gem

It doesn't work with Ruby 3.1+, and in 2018 its maintainer wrote that
users should consider switching to RuboCop.

Rakefile

@@ -4,20 +4,14 @@ ENV['PATH'] = "/opt/docker/:#{ENV['PATH']}" if ENV['CI'] == 'true'
 
 require 'docker'
 require 'rspec/core/rake_task'
-require 'cane/rake_task'
-
 
 desc 'Run the full test suite from scratch'
-task :default => [:unpack, :rspec, :quality]
+task :default => [:unpack, :rspec]
 
 RSpec::Core::RakeTask.new do |t|
   t.pattern = 'spec/**/*_spec.rb'
 end
 
-Cane::RakeTask.new(:quality) do |cane|
-  cane.canefile = '.cane'
-end
-
 desc 'Download the necessary base images'
 task :unpack do
   %w( swipely/base registry busybox:uclibc tianon/true debian:stable ).each do |image|

TESTING.md

@@ -33,9 +33,6 @@ This repository comes with five Rake commands to assist in your testing of the c
 ## `rake rspec`
 This command will run Rspec tests normally on your local system. You must have all the required base images pulled.
 
-## `rake quality`
-This command runs a code quality threshold checker to hinder bad code.
-
 ## `rake unpack`
 Pulls down all the required base images for testing.
 

docker-api.gemspec

@@ -15,7 +15,6 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency 'rake'
   gem.add_development_dependency 'rspec', '~> 3.0'
   gem.add_development_dependency 'rspec-its'
-  gem.add_development_dependency 'cane'
   gem.add_development_dependency 'pry'
   gem.add_development_dependency 'single_cov'
   gem.add_development_dependency 'webmock'