ci: dockerfile: avoid being asked question during apt-get

integration-tests/Dockerfile

@@ -2,7 +2,10 @@
 ARG ubuntu_version=20.04
 FROM ubuntu:${ubuntu_version}
 
-RUN apt-get update \
+# DEBIAN_FRONTEND is set to avoid being asked for input and hang during build:
+# https://anonoz.github.io/tech/2020/04/24/docker-build-stuck-tzdata.html
+RUN DEBIAN_FRONTEND=noninteractive \
+ && apt-get update \
  && apt-get install --yes \
         systemd \
         curl \