The short version is that everything is fine, you can ignore the message, nothing extra needs to be done.
The message means that the package can’t be pre-configured before its installation, because
apt-extracttemplates is missing. Nothing bad happens as a result, and the package does end up being configured correctly.
In more detail, when
apt-extracttemplates (part of
apt-utils) isn’t present, this is what happens:
apt builds up the set of packages to be installed, removed, etc.;
dpkg is invoked to perform the actual change;
- each package is extracted, and its maintainer scripts are run, including any configuration; in interactive mode, this can result in multiple questions being asked throughout installation.
apt-extracttemplates is present, package configuration can instead be performed early in the
dpkg run, before the packages are even extracted; this allows any questions to be asked ahead of time, and then installation can proceed without human intervention.
In a container context, installations are supposed to proceed in noninteractive mode anyway, so pre-extracting configuration templates offers no benefit and
apt-utils isn’t useful.
apt-utils’s purpose is described in its package description:
This package contains some less used commandline utilities related to package management with APT.
apt-extracttemplates is used by
debconf to prompt for configuration
questions before installation.
apt-ftparchive is used to create Packages and other index files
needed to publish an archive of Debian packages
apt-sortpkgs is a Packages/Sources file normalizer.