debconf attempts interactive configuration; breaks Docker image builds

[Jérôme Petazzoni]

When I build a container image using a Dockerfile based on the official "ubuntu" image, if I try to install packages that use debconf to obtain configuration information, the build fails because debconf tries to interact with the user (which is not possible when building a container, since the build process must be non-interactive).

How to reproduce:

1) install Docker (the version doesn't seem to matter; tried with various 19.03 variants)

2) run the following command to try and build an image:

```
docker build - <<EOF
FROM ubuntu
RUN apt-get update
RUN apt-get install -y perl-modules tzdata
EOF
```

The build remains stuck when debconf ask the "Geographic area:".

Notes:

A) The problem doesn't appear when trying to install tzdata by itself (without perl-modules), because then debconf hits a bug and bails out with a zero status code, allowing the package install to succeed:

```
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/DbDriver/Stack.pm line 111.
```

B) If you wonder "why is this person trying to install tzdata in a container?!?" - I'm not! But tzdata is a dependency of many other packages. When trying to reproduce the bug, I reduced it to the simplest case I could find (which was tzdata + perl-modules).

C) That problem did not happen in 16.04 (you can check by replacing "FROM ubuntu" with "FROM ubuntu:16:04" in the repro above). It did happen in 18.04, but it was less likely to show up, because tzdata was less depended upon. I have Dockerfiles that worked with 18.04, and break with 20.04, because they install packages that didn't depend on tzdata in 18.04, but do depend on tzdata in 20.04.

[Tianon Gravi (tianon)]

Hi Jérôme!

The upstream Debian bug is at https://bugs.debian.org/929417 (although unfortunately, no maintainer response).

[Benjamin Drung (bdrung)]

The Debian bug #929417 is fixed in debconf 1.5.81 and therefore will be in Ubuntu 23.04 (lunar).

The "Use of uninitialized value" warning is caused by the separate bug #1899343.