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)

`matrix_container_images_self_build` was not really doing anything
anymore. It previously was influencing `matrix_*_self_build` variables,
but it's no longer the case since some time ago.

Individual `matrix_*_self_build` variables are still available.
People that would like to toggle self-building for a specific component
ought to use those.

These variables are also controlled automatically (via
`group_vars/matrix_servers`) depending on `matrix_architecture`.

In other words, self-building is being done automatically for
all components when they don't have a prebuilt image for the specified
architecture. Some components only support `amd64`, while others also
have images for other architectures.

Related to https://github.com/vector-im/riot-web/pull/13914

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).

Fixes #523 (Github Issue).

Depending on the distro, common commands like sleep and chown may either
be located in /bin or /usr/bin.

Systemd added path lookup to ExecStart in v239, allowing only the
command name to be put in unit files and not the full path as
historically required. At least Ubuntu 18.04 LTS is however still on
v237 so we should maintain portability for a while longer.

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).

This will not work yet, as no version of Riot currently supports it.
It's expected to land in riot-web v1.5.16 via matrix-org/matrix-react-sdk#4348.

https://github.com/matrix-org/matrix-doc/pull/1957

Yay Riot iOS now supports integration manager discovery!

Riot used to be fine with it being blank but now it complains. This creates an ugly looking comma when there is an identity server configured but I guess that's fine.

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