Docker: if installed via a snap, check the allowed paths

Similar to what we do for docker machine users

https://github.com/common-workflow-language/cwltool/blob/57f9be0ae50e3f66ef4f6c5de430679e4390c48b/cwltool/docker.py#L31

https://snapcraft.io/docker tells us that only paths in `$HOME` are allowed

> I figured out the problem from [seveval posts](https://github.com/moby/moby/issues/34427#issuecomment-455828847) and discussion with my classmates.
Something always went wrong with `snap` installed Docker, but `apt` installed version works.

_Originally posted by @DawnEve in https://github.com/common-workflow-language/common-workflow-language/issues/927#issuecomment-917662005_

Start by reading cwltool/docker.py around the linked line and compare its handling of Docker Machine users. Determine how Docker installed through snap restricts accessible paths to those under $HOME. Done means snap-installed Docker paths are checked or handled consistently with the existing Docker Machine logic.

Written by the indexing model from the issue text.