Sign our images

When we push images to DockerHub we need to sign them. We should "likely" use some way to sign them with Github Key (because this is where we build the images) or change our process of releasing the image to allow signing by "release manager" - Docker Image is a convenience package, so we do not (currently) have requirement from the ASF to sign them and to use specific key, so for now we can choose whatever form of it is convenient to us