add initial ruby-based rspec test suite

* Use rubocop to ensure ruby follows consistent style
* Use rspec to check wormhole (devenv) dockerfile

Initial output is...

    Running RuboCop...
    Inspecting 2 files
    ..

    2 files inspected, no offenses detected
    /usr/bin/ruby -S rspec ./spec/wormhole/dockerfile_spec.rb

    wormhole (devenv) image
      should be available
      should expose ssh port and only ssh port
      should run sshd with logging

    Garbage collector stats (see spec_helper.rb for helpful links)
    --------------------------------------------------------------
    {:count=>20,
     :heap_used=>248,
     :heap_length=>248,
     :heap_increment=>0,
     :heap_live_num=>79996,
     :heap_free_num=>53456,
     :heap_final_num=>0,
     :total_allocated_object=>413398,
     :total_freed_object=>333402}

    Top 3 slowest examples (0.01076 seconds, 100.0% of total time):
      wormhole (devenv) image should be available
        0.00475 seconds ./spec/wormhole/dockerfile_spec.rb:13
      wormhole (devenv) image should expose ssh port and only ssh port
        0.00316 seconds ./spec/wormhole/dockerfile_spec.rb:17
      wormhole (devenv) image should run sshd with logging
        0.00284 seconds ./spec/wormhole/dockerfile_spec.rb:21

    Finished in 0.09239 seconds
    3 examples, 0 failures