dashboard/coordinator: run background goroutine cleaning old docker containers
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 <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

runcom/tools,glasser/golang-tools,josharian/tools,maddyonline/tools,runcom/tools,syreclabs/go-tools,dmitris/tools,springning/tools,matryer/tools,glasser/golang-tools,magiccao/tools,syreclabs/go-tools,nicot/tools,cw2018/tools,golang/tools,magiccao/tools,cmars/tools,cmars/tools,4ad/tools,sshetty10/tools,cmars/tools,matryer/tools,josharian/tools,magiccao/tools,magiccao/tools,nicot/tools,glasser/golang-tools,golang/tools,nicot/tools,matryer/tools,glasser/golang-tools,argandas/tools,syreclabs/go-tools,josharian/tools,cw2018/tools,maddyonline/tools,argandas/tools,dmitris/tools,jdhenke/tools,4ad/tools,argandas/tools,cw2018/tools,argandas/tools,syreclabs/go-tools,syreclabs/go-tools,springning/tools,jdhenke/tools,springning/tools,syreclabs/go-tools,cw2018/tools,maddyonline/tools,maddyonline/tools,golang/tools,nicot/tools,golang/tools,4ad/tools,sshetty10/tools,runcom/tools,jdhenke/tools,springning/tools,cmars/tools,matryer/tools,sshetty10/tools,dmitris/tools,jdhenke/tools,golang/tools,runcom/tools,dmitris/tools,sshetty10/tools,josharian/tools