Documentation/op-guide: add gcr.io image as alternative
Documentation/op-guide: add gcr.io image as alternative

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

fanminshi/etcd,YuleiXiao/etcd,mkumatag/etcd,gyuho/etcd,bdarnell/etcd,whuwxl/etcd,coreos/etcd,joshix/etcd,xiang90/etcd,visheshnp/etcd,li-ang/etcd,sinsharat/etcd,ajityagaty/etcd,YuleiXiao/etcd,bluebreezecf/etcd,bdarnell/etcd,coreos/etcd,etcd-io/etcd,etcd-io/etcd,akauppi/etcd,mkumatag/etcd,mkumatag/etcd,etcd-io/etcd,li-ang/etcd,heyitsanthony/etcd,ajityagaty/etcd,sebrandon1/etcd,rnaveiras/etcd,joshix/etcd,cockroachdb/etcd,fanminshi/etcd,sinsharat/etcd,a-robinson/etcd,lorneli/etcd,rnaveiras/etcd,visheshnp/etcd,lorneli/etcd,vimalk78/etcd,heyitsanthony/etcd,glevand/coreos--etcd,mqliang/etcd,akauppi/etcd,a-robinson/etcd,gyuho/etcd,whuwxl/etcd,vimalk78/etcd,xiang90/etcd,dannysauer/etcd,mqliang/etcd,bluebreezecf/etcd,rnaveiras/etcd,mqliang/etcd,ajityagaty/etcd,dannysauer/etcd,mickep76/etcd,mickep76/etcd,sebrandon1/etcd,cockroachdb/etcd,glevand/coreos--etcd