Add a dockerfile

[markstory]

Add the ability to build a container that can be used to run docket. Refs #57

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.07%. Comparing base (cb16392) to head (0bbd5de).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #58   +/-   ##
=========================================
  Coverage     92.07%   92.07%           
  Complexity      864      864           
=========================================
  Files           170      170           
  Lines          8602     8602           
=========================================
  Hits           7920     7920           
  Misses          682      682           

Flag	Coverage Δ
flutter	90.60% <ø> (ø)
phpunit	93.34% <ø> (ø)

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