This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#package-latest

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Discussed here: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1510

Future maintainer: check on https://docs.docker.com/engine/install/debian/ if Docker for
Debian 11 is released, then undo this commit

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.

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

Should help with #300 (Github Issue).

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