Support retrieving docker instances via 'docker import'
Support retrieving docker instances via 'docker import'

bcbio stores its container as a gzipped download via S3 because it's too big for the docker index. This PR allows pulling in instances similar to docker pull, but via docker import.

SciDAP/cwltool,jeremiahsavage/cwltool,common-workflow-language/cwltool,SciDAP/cwltool,common-workflow-language/cwltool,jeremiahsavage/cwltool,dleehr/cwltool,chapmanb/cwltool,dleehr/cwltool,jeremiahsavage/cwltool,dleehr/cwltool,chapmanb/cwltool,chapmanb/cwltool,SciDAP/cwltool,common-workflow-language/cwltool,SciDAP/cwltool,chapmanb/cwltool,dleehr/cwltool,jeremiahsavage/cwltool