Delay a build till previous build completes

Hey everyone,

I am trying to implement blue-green deployment pattern for my infrastructrue, which basically means I have 2 stacks on rancher for my application, one that is currently active(green) and other that is standby(blue). To do this , I need to ensure that builds happen serially,i.e. no build is scheduled till the previously running build is completed. Is there any way in drone with which I can make sure of this?