Add support for mountPath as directory

[41ks]

Description

This adds support for specifying a directory as the mountPath for a device type.

Use case

This is useful for situations in which the path variable includes a wildcard matching multiple devices which then need to be mounted in a different path in the container. For example:
Host FS:

/dev
    ttyUSB0
    ttyUSB1
    ttyUSB2

This feature would allow us to mount those devices to a different directory such as /dev/serial/:
Container FS:

/dev/serial
    ttyUSB0
    ttyUBS1
    ...

[41ks]

Thank you for the review @squat ! I applied your suggestion.

[squat]

I think this makes sense 👍 We might eventually choose to move this functionality into a separate, explicit field, like mountDirectory or something, to not complicate the logic of the mountPath option with magic.