Update deprecated apt-key command

deb.mk

@@ -19,7 +19,7 @@ install-from-deb:
 	wget -nv -O - https://get.docker.com/ | sh
 
 	@echo "--> Installing dokku"
-	wget -nv -O - https://packagecloud.io/dokku/dokku/gpgkey | apt-key add -
+	wget -qO- https://packagecloud.io/dokku/dokku/gpgkey | sudo tee /etc/apt/trusted.gpg.d/dokku.asc
 	@echo "deb https://packagecloud.io/dokku/dokku/ubuntu/ $(shell lsb_release -cs 2>/dev/null || echo "bionic") main" | sudo tee /etc/apt/sources.list.d/dokku.list
 	sudo apt-get update -qq >/dev/null
 	sudo DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true apt-get -qq -y --no-install-recommends install dokku