noninteractive frontend so tzdata doesn't ask for input

Author: electricbrass

ci/ubuntu-focal.Dockerfile

@@ -6,6 +6,7 @@ COPY . .
 
 # Packages - first the majority of them, then cmake
 RUN set -x && \
+    DEBIAN_FRONTEND=noninteractive \
     apt update && \
     apt install -y ninja-build libsdl2-dev libsdl2-mixer-dev \
         libpng-dev libcurl4-openssl-dev libwxgtk3.0-gtk3-dev deutex \