Docker feature doesn't work on macOS

[nsheff]

reported in several related issues:

• https://github.com/databio/bulker/issues/56
• https://github.com/refgenie/refgenie/issues/175

these lines are optimized for linux: https://github.com/databio/pypiper/blob/67908f2ee5f51fa5fdddb67eb6d7891aefeeda6a/pypiper/manager.py#L1904-L1908

need to somehow solve for macos natively.

[nsheff]

workaround posted in https://github.com/databio/bulker/issues/56#issuecomment-677708788