Continuation of 1fca917ad1.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/722

`-v` magically creates the source destination as a directory,
if it doesn't exist already. We'd like to avoid this magic
and the potential breakage that it might cause.

We'd rather fail while Docker tries to find things to `--mount`
than have it automatically create directories and fail anyway,
while having contaminated the filesystem.

There's a lot more `-v` instances remaining to be fixed later on.
This is just some start.

Things like `matrix_synapse_container_additional_volumes` and
`matrix_nginx_proxy_container_additional_volumes` were not changed to
use `--mount`, as options for each one are passed differently
(`ro` is `ro`, but `rw` doesn't exist and `slave` is `bind-propagation=slave`).
To avoid breaking people's custom volume mounts, we keep it as it is for now.

A deficiency with `--mount` is that it lacks the `z` option (SELinux
ownership changes), and some of our `-v` instances use that. I'm not
sure how supported SELinux is for us right now, but it might be,
and breaking that would not be a good idea.

Also switches to using `docker system prune -a` for a less invasive
cleanup of Docker images and related resources.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/727

Related to #724 (Github Pull Request)

This reverts commit 078592454c
due to reports of breakage both in the support chat room
and in here https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/719

This is both for consistency with 93cc71cb69976c
and for making things more obvious.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/716

This patch makes us use more fully-qualified container image names
(either prefixed with docker.io/ or with localhost/).

The latter happens when self-building is enabled.

We've recently had issues where if an image was removed manually
and the service was restarted (making `docker run` fetch it from Docker Hub, etc.),
we'd end up with a pulled image, even though we're aiming for a self-built one.
Re-running the playbook would then not do a rebuild, because:
- the image with that name already exists (even though it's something
else)
- we sometimes had conditional logic where we'd build only if the git
repo changed

By explicitly changing the name of the images (prefixing with localhost/),
we avoid such confusion and the possibility that we'd automatically pul something
which is not what we expect.

Also, I've removed that condition where building would happen on git
changes only. We now always build (unless an image with that name
already exists). We just force-build when the git repo changes.

Changelog https://github.com/jitsi/jitsi-meet/releases/tag/stable%2Fjitsi-meet_5142

Regression since #681 (Github Pull Request).

Fixes #715 (Github Issue).

We'd like the roles to be self-contained (as much as possible).

Thus, the `matrix-nginx-proxy` shouldn't reference any variables from
other roles. Instead, we rely on injection via
`group_vars/matrix_servers`.

Related to #681 (Github Pull Request)

If `matrix_dynamic_dns_enabled`, we'd like to ensure there's at least
one configuration defined.

Related to #681 (Github Pull Request)

- `item` was undefined
- `'key' in configurations == ''` was doing the wrong thing

Related to #681 (Github Pull Request)

Having it unset in the role itself (while referencign it) is a little strange.

Now people can look at the `roles/matrix-dynamic-dns/defaults/main.yml`
file and figure out everything that's necessary to run the role.

Related to #681 (Github Pull Request)

(quoting ftw)

so transfer limits are properly set in accord to the relevant setting
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/456#issuecomment-719996778

(allegedly breaks with SSO enabled)

to prevent "404 on the federation port for the path `/_matrix/media`,
if a remote server is trying to get the media object on federation
port, see https://github.com/matrix-org/synapse/issues/8695 "

https://github.com/matrix-org/synapse/pull/8701

This broke in 63a49bb2dc.

Proxying the OpenID Connect endpoints is now possible,
but needs to be enabled explicitly now.

Supersedes #702 (Github Pull Request).

This patch builds up on the idea from that Pull Request,
but does things in a cleaner way.

Fixes #696 (Github Issue)

We do this to match Synapse's new default "max_upload_size" (50MB).

This `matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb`
default value only affects standalone usage of the `matrix-nginx-proxy`
role. When the role is used in the context of the playbook,
the value is dynamically assigned from `group_vars/matrix_servers`.

Somewhat related to #692 (Github Issue).