`apt purge` does not remove /var/lib/docker

[DarkTrick]

Preface
========
- I'm filing this against docker.io, but it might as well be a problem with apt.
- I experienced this with the docker.io package, but I can imagine, that there are other packages out there, that have exactly the same problem. Please read this report as an example for a perhaps generic problem.

What happened:
===============
- `apt purge docker.io` does not remove docker's file directory `/var/lib/docker`. Therefore previously used images and other files will be left on the system. This might be expected using `apt remove`, but `apt purge` should remove "config files" as well.
- The sysadmin does not get any messages, that there are files left on the system.

Generic Problem:
================
apt-get is supposed to manage files on a ubuntu machine. It fails to meet its goal in this case.

Concrete description:
======================
- This might be a difficult case, as docker images can be seen as "user files" (not config files). It would not be the right thing to remove those files.
- The general user does probably not know about this directory. This is a very potential file graveyard on the system.
- Upon installing no one tells the admin (in a clear, informative way), that there will be files create in `/var/lib/docker`. The information is hidden from the user. In that case, apt should be responsible for doing something about these files, if they get deleted.

Suggestions / Solution
========================
(I don't have *the* solution for this problem. I guess it's necessary to discuss what's the right thing to do)

My idea: Put them in $HOME/docker (or similar) instead of /var/lib/docker. (yes, they might be seen as "system wide user files", but I think real admins know how they could config paths and change the location while simple users immediately see that there's a `docker` folder in their home directory)

Ubuntu 20.10

docker.io:
  Installed: (none)
  Candidate: 19.03.13-0ubuntu3
  Version table:
     19.03.13-0ubuntu3 500
        500 http://archive.ubuntu.com/ubuntu groovy/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: docker.io (not installed)
ProcVersionSignature: Ubuntu 5.8.0-32.34-generic 5.8.18
Uname: Linux 5.8.0-32-generic x86_64
ApportVersion: 2.20.11-0ubuntu50.2
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Sun Dec 6 08:00:37 2020
SourcePackage: docker.io
UpgradeStatus: No upgrade log present (probably fresh install)

[Tianon Gravi (tianon)]

This confuses me, because this is already implemented?

- https://git.launchpad.net/ubuntu/+source/docker.io/tree/debian/docker.io.prerm?h=applied/ubuntu/devel
- https://git.launchpad.net/ubuntu/+source/docker.io/tree/debian/docker.io.postrm?h=applied/ubuntu/devel

It was added ~four years ago in 1.11.2-0ubuntu6:

  docker.io (1.11.2-0ubuntu6) yakkety; urgency=medium

    ...
      - remove "/var/lib/docker" on purge
    ...

   -- Tianon Gravi <email address hidden> Fri, 19 Aug 2016 10:15:14 -0700

It sounds like maybe there's a bug where this isn't working correctly in all cases?

[Tianon Gravi (tianon)]

Just to verify, I installed docker.io version 19.03.8-0ubuntu1.20.04.1, started up dockerd, verified /var/lib/docker got some content in it, and then stopped dockerd and ran "apt purge docker.io", and "/var/lib/docker" was properly emptied. It was not removed completely (in case it's a mount point or something similar), but there's no content in it (left as just an empty directory).