Default Incoming path incorrectly separated as `/app/\Incoming` on Linux (i.e. Docker image)

[jimbobmcgee]

Describe the bug
The default location for stored incoming mail (and the log file) is intended to be /app/Incoming but is being incorrectly concatenated as /app/\Incoming (note extra \). This makes accessing that directory (e.g. for bind-mounting a local volume) more awkward than it should be, as the extra backslash has to be escaped in most Linux tools.

To Reproduce
No repo steps; default behaviour with current Docker image

Expected behavior
Paths should always be built with the OS-specific path separator, e.g. using System.IO.Path.Combine or System.IO.Path.DirectorySeparatorChar

Screenshots

Desktop (please complete the following information):
Deployed from docker.io/changemakerstudiosus/papercut-smtp

Additional context
Default config file (which contains "MessagePath": "%BaseDirectory%\\Incoming") may need to be OS-specific, or pre-processing may need to be done on the config-supplied value to normalize path separators and re-emit with OS-specific separators.