add rake to Gemfile

Gemfile

 source "https://rubygems.org"
 
 group :development do
+  gem "rake"
   gem "docker-api"
   gem "rspec"
   gem "concurrent-ruby"

Gemfile.lock

@@ -9,6 +9,7 @@ GEM
       json
     excon (0.45.3)
     json (1.8.2)
+    rake (10.4.0)
     ref (1.0.5)
     rspec (3.2.0)
       rspec-core (~> 3.2.0)
@@ -30,6 +31,7 @@ PLATFORMS
 DEPENDENCIES
   concurrent-ruby
   docker-api
+  rake
   rspec
 
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