added docker ignore file to ignore work and output directories that may exist
added docker ignore file to ignore work and output directories that may exist

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