I've set the following environment so that no question/dialog is asked during apt-get install:
ENV DEBIAN_FRONTEND noninteractive # export DEBIAN_FRONTEND="noninteractive"
Which is equivalent to:
Yet, when building an image from a Dockerfile, at the end of one specific Debian/Ubuntu package install (using apt-get install), package configuration debconf says:
debconf: unable to initialize frontend: Noninteractive # export DEBIAN_FRONTEND="noninteractive" debconf: (Bareword "Debconf::FrontEnd::Noninteractive" not allowed while "strict subs" in use at (eval 35) line 3, <> line 1.) debconf: falling back to frontend: Noninteractive Subroutine BEGIN redefined at (eval 36) line 2, <> line 1.
Wow... I found my error. There's no way to put comment in ENV lines in Dockerfile. I'm going to answer myself as I'm sure this will bite some other people...