Add warning on docker group suggestion

Signed-off-by: Eric Curtin <ericcurtin17@gmail.com>

hack/install.sh

@@ -78,6 +78,12 @@ echo_docker_as_nonroot() {
 
 	Remember that you will have to log out and back in for this to take effect!
 
+	WARNING: Adding a user to the "docker" group will grant the ability to run
+	         containers which can be used to obtain root privileges on the
+	         docker host.
+	         Refer to https://docs.docker.com/engine/security/security/#docker-daemon-attack-surface
+	         for more information.
+
 	EOF
 }