Docker.io upgrade ignores -y and hangs in readline

[Pehr Söderman]

Running a remote upgrade of docker.io results in a hang after it posts a readline question despite running with -y:

Setting up docker.io (20.10.7-0ubuntu5~20.04.1) ...
debconf: unable to initialize frontend: Dialog
debconf: (Dialog frontend will not work on a dumb terminal, an emacs shell buffer, or without a controlling terminal.)
debconf: falling back to frontend: Readline
Configuring docker.io
---------------------

If Docker is upgraded without restarting the Docker daemon, Docker will often
have trouble starting new containers, and in some cases even maintaining the
containers it is currently running. See https://launchpad.net/bugs/1658691 for
an example of this breakage.

Normally, upgrading the package would simply restart the associated daemon(s).
In the case of the Docker daemon, that would also imply stopping all running
containers (which will only be restarted if they're part of a "service", have an
appropriate restart policy configured, or have some other means of being
restarted such as an external systemd unit).

Automatically restart Docker daemon? [yes/no] no

<*hangs regardless of response*>

I would expect the install to continue here with the default answer (-y).