[docs] Update builder docs on last-modified times
[docs] Update builder docs on last-modified times

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

riyazdf/cli,riyazdf/cli,dnephin/cli,e11137/cli,e11137/cli,cpuguy83/cli,Zebrilee/cli,cpuguy83/cli,docker/cli,docker/cli,allencloud/cli,ashfall/cli,allencloud/cli,thaJeztah/cli,Zebrilee/cli,ashfall/cli,redpanda/cli,redpanda/cli,thaJeztah/cli,dnephin/cli