Mount directories outside of working dir in cog run

[andreasjansson]

It's currently impossible to mount directories outside of the working directory when doing cog run.

It's common to keep datasets in separate folders than the working directory when training models, so in order for Cog to be a viable ML dev tool it ought to support mounting directories.

[simonmaris]

I second this, it would be very useful, we store models on another location as well. An option to follow symlinks in the src folder would work as well :)