This also adds support for installing python3-docker (not python-docker)
in systems that run Python 3.

Raspbian doesn't seem to support arm64, so this is somewhat pointless
right now.

However, they might in the future. Doing this should also unify us
some more with `setup_debian.yml` with the ultimate goal of
eliminating `setup_raspbian.yml`.

It's likely installed by default, but it doesn't hurt to specify it.
It also makes us more the same with `setup_debian.yml`.

Well, `ubuntu` or `debian`, the same key is served right now,
so it doesn't really matter.

This seems cleaner and less prone to breakage though.

Until now, we've only supported non-amd64 on Raspbian.

Seems like there are now people running Debian/Ubuntu on ARM,
so we were forcing them into amd64 Docker packages.

I've gotten a report that this change fixes support
for Ubuntu Server 20.04 on RPi 4B.

Interestingly, no one has reported this failure before #662 (Github
Issue).

It doesn't make sense to keep saying that we support such old Ansible
versions, when we're not even testing on anything close to those.

Time is also passing and such versions are getting more and more
ancient. It's time we bumped our requirements to something that is more
likely to work.

I don't believe Ansible v1 would even go as far as executing this
sanity check, but.. Adding an extra defensive check for completeness.

Certain more-minimal Debian installations may not have
lsb-release installed, which makes the playbook fail.

We need lsb-release on Debian, so that ansible_lsb
could tell us if this is Debian or Raspbian.

AWS Debian marketplace image does not have gpg preinstalled

https://aws.amazon.com/marketplace/pp/B0859NK4HC?ref=cns_srchrow

TASK [matrix-base : Ensure Docker's APT key is trusted] *******************************************************************************************************************************************************
fatal: [matrix.domain.com]: FAILED! => {"changed": false, "msg": "Failed to find required executable gpg in paths: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"}

Closes #590

Fixes #586 (Github Issue)

Certain people organize their inventory in a different way
and we'd like to accommodate them.

Related to #542 (Github Issue).

Fixes #542 (Github Issues).

We don't really need to fail in such a spectactular way,
but it's probably good to do. It will only happen for people
who are defining their own user/group id, which is rare.

It seems like a good idea to tell them that this doesn't work
as they expect anymore and to ask them to remove these variables,
which otherwise give them a fake sense of hope.

Related to #486 (Github Pull Request).

If one runs the playbook with `--tags=setup-all`, it would have been
fine.

But running with a specific tag (e.g. `--tags=setup-riot-web`) would
have made that initialization be skipped, and the `matrix-riot-web` role
would fail, due to missing variables.

No migration steps should be required.

Should help with #300 (Github Issue).

We've just updated it in the task above, so it's unnecessary

We've just updated it in the task above, so it's unnecessary