Note to self: Currently the CI will have to download & install the whole system, in addition to fixed versions of ANTs and FSL, for every CI test. An alternative would have to be a pre-built container based on an additional repository, which includes all of these contents: this repository would then simply specify this container in the Dockerfile recipe. The disadvantange would be that updates to other neuroimaging softwares would not immediately be imported; but this would simply require updating and rebuilding that template container.

  Step 10/16 : RUN wget -q http://fsl.fmrib.ox.ac.uk/fsldownloads/fslinstaller.py -O /fslinstaller.py     && chmod 775 /fslinstaller.py     && python2 /fslinstaller.py -d /opt/fsl -V 6.0.4 -q     && rm -f /fslinstaller.py     && ( which immv || ( rm -rf /opt/fsl/fslpython && /opt/fsl/etc/fslconf/fslpython_install.sh -f /opt/fsl || ( cat /tmp/fslpython*/fslpython_miniconda_installer.log && exit 1 ) ) )
   ---> Running in 40f6e1aec579
  [FAILED] Unable to unpack FSL - Unable to unpack FSL.
  The command '/bin/sh -c wget -q http://fsl.fmrib.ox.ac.uk/fsldownloads/fslinstaller.py -O /fslinstaller.py     && chmod 775 /fslinstaller.py     && python2 /fslinstaller.py -d /opt/fsl -V 6.0.4 -q     && rm -f /fslinstaller.py     && ( which immv || ( rm -rf /opt/fsl/fslpython && /opt/fsl/etc/fslconf/fslpython_install.sh -f /opt/fsl || ( cat /tmp/fslpython*/fslpython_miniconda_installer.log && exit 1 ) ) )' returned a non-zero code: 1
  ##[warning]Docker build failed with exit code 1, back off 8.871 seconds before retry.
  /usr/bin/docker build -t 3b3ac6:84564b29c60e43bfb8327824a5fd2027 -f "/home/runner/work/_actions/MRtrix3/script_test_action/master/Dockerfile" "/home/runner/work/_actions/MRtrix3/script_test_action/master"
  Sending build context to Docker daemon  8.704kB
  
  Error response from daemon: Error processing tar file(exit status 1): write /action.yml: no space left on device

:-/

Container grabs code, compiles and starts tests, but they get terminated after 6 hours.
So will need to pursue test data drastically reduced in spatial resolution to complete more quickly, and/or Daan's suggestion of replacing external software commands with dummies that dump pre-generated data; but given population_template is one of the culprits, reduction in spatial resolution is probably going to be needed regardless.

@maxpietsch: I've massively cut down on dwifslpreproc test run times trying to make CI tests with little to no multi-threading and 6 hour runtime feasible (and hopefully actually add more tests to verify all of its features). Ideally I'd like to cut down on population_template substantially too since that's the other primary culprit. There's a few things we could do e.g.:

• Reduce spatial resolution;

• Reduce the number of tests that are done comprehensively looking for regressions in output data; for those that only need to be run to verify completion given command-line options, we could cut number of stages / number of iterations within each stage.

Though CI is actually out of time before the tests even reach that point, so there needs to be further cuts elsewhere...