Fix Dockerfile that was missing LICENSE file

[takluyver]

The build stage of the Dockerfile was trying to create just enough of the source folder to install the dependencies. This broke on a new version of Flit because the LICENSE file wasn't included. But I don't think there's any problem with just copying the whole git repo folder in, like the app stage does later. So this simplfiies it.

@RobertRosca what do we gain by installing all the dependencies in the build stage and then installing DAMNIT itself separately in the app stage? Could we just install it in the build stage and then copy it across as part of /app?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.24%. Comparing base (17e0755) to head (151c01f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #405   +/-   ##
=======================================
  Coverage   76.24%   76.24%           
=======================================
  Files          35       35           
  Lines        6063     6063           
=======================================
  Hits         4623     4623           
  Misses       1440     1440           

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[tmichela]

Thanks! LGTM
Maybe you can also revert 21baa24 where I lower the flit version.