fix: docker build warnings

[mgxd]

Address the following:

 9 warnings found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 74)
 - UndefinedVar: Usage of undefined variable '$PYTHONPATH' (line 191)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 40)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 98)
 - UndefinedVar: Usage of undefined variable '$FSLDIR' (line 175)
 - UndefinedVar: Usage of undefined variable '$LD_LIBRARY_PATH' (line 191)
 - UndefinedVar: Usage of undefined variable '$CPATH' (did you mean $PATH?) (line 207)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 35)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 51)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.19%. Comparing base (3b4d6e9) to head (c8eb353).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #495   +/-   ##
=======================================
  Coverage   57.19%   57.19%           
=======================================
  Files          70       70           
  Lines        6866     6866           
  Branches      848      848           
=======================================
  Hits         3927     3927           
  Misses       2723     2723           
  Partials      216      216           

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