I can only confirm this issue. apt-utils is a Priority: important package, should be installed by default.

The set of installed packages is controlled by Canonical in the tar files they generate for the "minimal set of packages that can be called Ubuntu": https://partner-images.canonical.com/core/ and we only add a few tweaks to make the docker images nicer.

Is there a way to contact them / open a ticket about why is this missing from partner-images?

Is there any harm (barring the warning) caused by not having this package installed? I think there might be a general "Ubuntu" bug tracker on launchpad, but I'm not sure.

http://serverfault.com/a/358950 implies that it only matters for interactive installations (ie, the installer needs to format a prompt template for display to the user), so it seems like it's really not necessary for the majority of Docker users (who install applications via a Dockerfile, and thus non-interactively).

Is there any harm (barring the warning) caused by not having this package installed?

No, it has not yet stopped anything from running. Just a warning, and nothing else.

it only matters for interactive installations

I read the answer before, but I did not know that "questions about how to be configured" is just a confirmation with size information provided (since I did not found any good online explanation for it). If that is the case, we need to open a ticket to tell Canonical to suppress the warning when apt-get is called with option --assume-yes.

I Googled for the Ubuntu bug tracker, and I found this guide. I think this bug belongs to the section Reporting non-crash hardware and desktop application bugs, but it suggests to report the bug with ubuntu-bug, which is (as expected) not installed in the minimal Ubuntu image; I get stuck here.

sum up

Currently best workaround is

ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install --assume-yes apt-utils

which gives a single line of warning

debconf: delaying package configuration, since apt-utils is not installed

After that, all the new installations will be free of those warnings.

Thanks for the excellent summary! Closing, since this isn't a bug (and causes no visible harm). 👍