Add support for running a command once the rclone drive is mounted

[seertenedos]

I recently found i was having bad performance partly due to the filesystem not being cached and that it currently takes a long time to cache if calling folder by folder but if instead you run an rclone command it can be done quickly. If you seen the service in the link below it runs "/usr/bin/rclone rc vfs/refresh recursive=true --rc-addr 127.0.0.1:5575 _async=true" to quickly get the filelist synced after it starts and i would like to be able to do the same things with your docker image.

https://github.com/animosity22/homescripts/blob/master/systemd/rclone-tv.service