fix(docker): exclude .dockerignore from context hash

[ethanwu10]

Since .dockerignore is a means to control what is in the build context, the contents of the file itself should not be included in the context hash. See #117 for more details.

Closes #117

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.70%. Comparing base (2107b0a) to head (db0fb48).
⚠️ Report is 33 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   60.70%   60.70%           
=======================================
  Files          27       27           
  Lines         990      990           
  Branches      167      167           
=======================================
  Hits          601      601           
  Misses        375      375           
  Partials       14       14           

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