Add DIPY_HOME environment variable to Dockerfile

[tsalo]

Closes #381.

Changes proposed in this pull request

• Set DIPY_HOME environment variable within Dockerfile so amico looks for downloaded files in that location instead of the home directory.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.44%. Comparing base (79953f5) to head (f828bcb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #382   +/-   ##
=======================================
  Coverage   50.44%   50.44%           
=======================================
  Files          55       55           
  Lines        8139     8139           
  Branches      944      944           
=======================================
  Hits         4106     4106           
  Misses       3794     3794           
  Partials      239      239           

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