Adjust absolute paths on Windows for Docker

[acornmakes]

The Windows Docker CLI has very specific requirements for absolute paths on Windows. The tool accepts absolute path formats such as /c/path/to/dest.

The Rust canonicalize function creates a UNC path which the Docker CLI command does not accept.

This PR addresses the issue #28 by adding a new function with a custom block of Windows only code (using cfg! macro) to reformat the path to a Docker CLI compatible path string. On other OSes, the string is returned as it was before, unchanged.