Update Dockerfile to use pipenv

[kirkins]

I went to use the Dockerfile and I noticed requirements.txt didn't exist anymore but there was a Pipfile.

So I updated to use that instead.

[codecov-io]

Codecov Report

Merging #1185 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1185   +/-   ##
=======================================
  Coverage   43.65%   43.65%           
=======================================
  Files          32       32           
  Lines        3569     3569           
  Branches      579      579           
=======================================
  Hits         1558     1558           
  Misses       1869     1869           
  Partials      142      142

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[ohld]

Wow. Great fix. Thanks!!!!