Document "scratch" mountpoint

[cirocosta]

Hey,

In concourse/concourse#1045, there was the initial work to have /scratch, a mountpoint that allows one to bypass overlay and have a direct mount to the underlying filesystem (that overlay sits on top of).

While that detail is leveraged by resources like docker-image-resource, that is not documented in ci.concourse-ci.org.

Having this mount point documented under our docs could then allow folks to more easily discover this future and work around problems that overlay might cause once we migrate from btrfs as a default preference in detect to overlay (see concourse/concourse#4071).

Thanks!

[taylorsilva]

This is a tough one to document. /scratch is made available for every container so it applies to everything in a pipeline. I'm going to pin this issue for now to help people discover it. I'm open to any suggestions people have for which page this info should live on.