fix: allow windows mounts to reference docker volumes

[RichieSams]

MountParser assumes that it's parsing a local absolute path for the src arg. However, it's also legal to use a named docker volume. This change allows for that case.

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[aeneasr]

Does that keep backwards compatibility? If so, could you reference this in some docs from docker? Thank you! :)

[RichieSams]

https://docs.docker.com/storage/bind-mounts/#choose-the--v-or---mount-flag
https://docs.docker.com/storage/volumes/#choose-the--v-or---mount-flag

From what I can tell from those docs, even what is put for non windows is incorrect.
The format is:

<src>:<dest>[:<options>]

options is optional.
For volume mounting, src is the name of a volume
For bind-mounts, src is the absolute path on the host

For bind-mounts on Windows, since src is required to be an absolute path, src will have an extra colon, due to the "drive".

I will update the PR accordingly

[RichieSams]

I found it difficult to try to re-capture all of the edge cases. So instead, I'm importing / using the same function used by docker-cli to parse the volume string.

[aeneasr]

That makes a ton of sense, thank you!