Run checks before execution logic
Run checks before execution logic

 - Check that docker program can be run
 - Verify docker can connect to daemon
 - Test for presence of local images

Also give useful help/version info when local image isn't present to
pass-throuh to

oesteban/fmriprep,oesteban/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/preprocessing-workflow