DOC: Run Singularity with `--cleanenv`

[shnizzedy]

Summary

List of changes proposed in this PR (pull-request)

• simplifies build recipe and supplies additional documentation based on https://github.com/shnizzedy/mindboggle/issues/4#issuecomment-812523157

Acknowledgment

• [x] (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov[bot]]

Codecov Report

Merging #9 (4a33585) into develop/v1.3.9 (bc07b2b) will not change coverage. The diff coverage is n/a.

@@               Coverage Diff               @@
##           develop/v1.3.9       #9   +/-   ##
===============================================
  Coverage           51.65%   51.65%           
===============================================
  Files                  42       42           
  Lines                6663     6663           
===============================================
  Hits                 3442     3442           
  Misses               3221     3221           

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