lift code from docker/volume/mounts for splitting windows volumes

[damoxc]

Using the API as provided from the mounts package imposes validation
on the src:dest which shouldn't be performed at this time. To workaround
that lift the internal code from that library required to only perform
the split.

Just wanted to share the patch I've used to fix Nomad for our use case, I'm not sure if this is the road you will want to down though.

A proposed fix for #5798.

[hashicorp-cla]

All committers have signed the CLA.

[notnoop]

Thanks @damoxc, this seems promising indeed ! I would suggest moving lifted code to a separate file and include a comment linking to original source along with license notice.

[damoxc]

@notnoop My pleasure! I've moved the code out to a new file and added some comments to the top to the source file and license in the Docker repository, is that what you were after?

[notnoop]

Thank you so much for your contribution! I'll follow up and update appveyor tests (which sadly don't run on PR builds).

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.