sdist build: when building docker image, no interactive prompts!

see https://askubuntu.com/questions/909277/

contrib/build-linux/sdist/Dockerfile

@@ -1,6 +1,7 @@
 FROM ubuntu:20.04@sha256:5747316366b8cc9e3021cd7286f42b2d6d81e3d743e2ab571f55bcd5df788cc8
 
 ENV LC_ALL=C.UTF-8 LANG=C.UTF-8
+ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update -q && \
     apt-get install -qy \