Suppress the warning: “unable to initialize frontend: Dialog” on Debian based images.

[mazgi]

My checklist

• I read CONTRIBUTING.md
• Search the issue repository to ensure your report is a new issue
• Recreate the issue after disabling all extensions
• Simplify your code around the issue to better isolate the problem

My environments

$ docker --version
Docker version 18.09.2, build 6247962
$ docker images debian:9
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
debian              9                   5a6d49d5e833        9 days ago          101MB

$ docker run -it debian:9 /bin/bash
root@3ef01a935271:/# env | grep -c DEBIAN_FRONTEND 
0
root@3ef01a935271:/# type dialog
bash: type: dialog: not found

The problem

We occurred the following warnings: debconf: unable to initialize frontend: Dialog.

root@e51d8207768e:/# apt-get update -qq
root@e51d8207768e:/# apt-get install --assume-yes --no-install-recommends apt-utils > /dev/null 
debconf: delaying package configuration, since apt-utils is not installed
root@e51d8207768e:/# apt-get install --assume-yes --no-install-recommends locales > /dev/null 
debconf: unable to initialize frontend: Dialog
debconf: (No usable dialog-like program is installed, so the dialog based frontend cannot be used. at /usr/share/perl5/Debconf/FrontEnd/Dialog.pm line 76, <> line 2.)
debconf: falling back to frontend: Readline
debconf: unable to initialize frontend: Readline
debconf: (Can't locate Term/ReadLine.pm in @INC (you may need to install the Term::ReadLine module) (@INC contains: /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.24.1 /usr/local/share/perl/5.24.1 /usr/lib/x86_64-linux-gnu/perl5/5.24 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.24 /usr/share/perl/5.24 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base .) at /usr/share/perl5/Debconf/FrontEnd/Readline.pm line 7, <> line 2.)
debconf: falling back to frontend: Teletype

Solution

• Install the dialog package each Debian based image.

And I think we should only unset the DEBIAN_FRONTEND environment variable, that sufficient to switch back to default frontend.

[msftclas]

All CLA requirements met.

[mazgi]

Oops... I cannot sign the CLA.
I'll try again after a few minutes.

[Chuxel]

These are great suggestions - thanks for the PR. Apologies for the delayed response, I wanted to fire off a full regression and take a look at the results before approving. All passed and LGTM!