chore: Use test container for testing fmriprep-docker

[effigies]

Not sure how we missed this in #3503, but we need to use a downloaded image to avoid reading stdin in fmriprep-docker.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.64%. Comparing base (cfe7a06) to head (adc5a0d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3520   +/-   ##
=======================================
  Coverage   73.64%   73.64%           
=======================================
  Files          60       60           
  Lines        4759     4759           
  Branches      616      616           
=======================================
  Hits         3505     3505           
  Misses       1111     1111           
  Partials      143      143           

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