Don't prompt user for time zone in during Docker build

See https://askubuntu.com/questions/909277/avoiding-user-interaction-with-tzdata-when-installing-certbot-in-a-docker-container

Dockerfile

@@ -1,6 +1,7 @@
 from ubuntu:18.04
 
 # Install prerequisites
+env DEBIAN_FRONTEND=noninteractive
 run apt-get update && apt-get install -y \
     build-essential \
     cmake \