dashboard/coordinator: run background goroutine cleaning old docker containers

I'm tired of figuring out what isn't cleaning up after itself, so keep
a background goroutine that looks at old containers and deletes them
as a backup measure. Verified it works by creating some dummy containers on
the machine.

Also adds df output to the HTML status page.

Change-Id: I23adc22872def882b3b9b3a4ec730017899bb966
Reviewed-on: https://go-review.googlesource.com/1537
Reviewed-by: Andrew Gerrand <adg@golang.org>